Proto-Indo-European phonology
Encyclopedia
The phonology
Phonology
Phonology is, broadly speaking, the subdiscipline of linguistics concerned with the sounds of language. That is, it is the systematic use of sound to encode meaning in any spoken human language, or the field of linguistics studying this use...

 of the Proto-Indo-European language
Proto-Indo-European language
The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 (PIE)
has been reconstructed
Linguistic reconstruction
Linguistic reconstruction is the practice of establishing the features of the unattested ancestor of one or more given languages. There are two kinds of reconstruction. Internal reconstruction uses irregularities in a single language to make inferences about an earlier stage of that language...

 by linguists, based on the similarities and differences among current and extinct Indo-European languages
Indo-European languages
The Indo-European languages are a family of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia...

. Because PIE was not written, linguists must rely on the evidence of its earliest attested descendants, such as Hittite
Hittite language
Hittite is the extinct language once spoken by the Hittites, a people who created an empire centred on Hattusa in north-central Anatolia...

, Sanskrit
Sanskrit
Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

, Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

, and Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 in order to reconstruct its phonology.

The reconstruction of abstract units of PIE phonological systems (i.e. segments, or phoneme
Phoneme
In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....

s in traditional phonology) is much less controversial than their phonetical interpretation. This especially pertains to the phonetic interpretation of PIE vowels, laryngeals
Laryngeal theory
The laryngeal theory is a generally accepted theory of historical linguistics which proposes the existence of one, or a set of three , consonant sounds termed "laryngeals" that appear in most current reconstructions of the Proto-Indo-European language...

 and voiced stops
Stop consonant
In phonetics, a plosive, also known as an occlusive or an oral stop, is a stop consonant in which the vocal tract is blocked so that all airflow ceases. The occlusion may be done with the tongue , lips , and &...

.

Phonemic inventory

Proto-Indo-European is traditionally reconstructed to have used the following phoneme
Phoneme
In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....

s. See the article on Indo-European sound laws
Indo-European sound laws
As the Proto-Indo-European language broke up, its sound system diverged as well, according to various sound laws in the daughter languages.Especially notable is the palatalization that produced the Satem languages, along with the associated ruki sound law...

 for a summary of how these phonemes reflected in the various Indo-European languages.

Consonants

Proto-Indo-European consonant segments
Labial
Labial consonant
Labial consonants are consonants in which one or both lips are the active articulator. This precludes linguolabials, in which the tip of the tongue reaches for the posterior side of the upper lip and which are considered coronals...

Coronal
Coronal consonant
Coronal consonants are consonants articulated with the flexible front part of the tongue. Only the coronal consonants can be divided into apical , laminal , domed , or subapical , as well as a few rarer orientations, because only the front of the tongue has such...

Dorsal
Dorsal consonant
Dorsal consonants are articulated with the mid body of the tongue . They contrast with coronal consonants articulated with the flexible front of the tongue, and radical consonants articulated with the root of the tongue.-Function:...

Laryngeal
Laryngeal theory
The laryngeal theory is a generally accepted theory of historical linguistics which proposes the existence of one, or a set of three , consonant sounds termed "laryngeals" that appear in most current reconstructions of the Proto-Indo-European language...

palatal plain labial
Nasal
Nasal consonant
A nasal consonant is a type of consonant produced with a lowered velum in the mouth, allowing air to escape freely through the nose. Examples of nasal consonants in English are and , in words such as nose and mouth.- Definition :...

} }
Plosive 
voiceless
} } } [k] } [q] }  
voiced (}) } } [ɡ] } [ɢ] }  
aspirated
Aspiration (phonetics)
In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ...

} } } [ɡʱ] } [ɢʱ] }  
Fricative
Fricative consonant
Fricatives are consonants produced by forcing air through a narrow channel made by placing two articulators close together. These may be the lower lip against the upper teeth, in the case of ; the back of the tongue against the soft palate, in the case of German , the final consonant of Bach; or...

}
Liquid
Liquid consonant
In phonetics, liquids or liquid consonants are a class of consonants consisting of lateral consonants together with rhotics.-Description:...

Semivowel
Semivowel
In phonetics and phonology, a semivowel is a sound, such as English or , that is phonetically similar to a vowel sound but functions as the syllable boundary rather than as the nucleus of a syllable.-Classification:...

} }


The table gives the most common notation in modern publications. Variant transcriptions are given below. Raised stands for aspiration
Aspiration (phonetics)
In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ...

.

Labials

PIE are conveniently grouped with the cover symbol P. The phonemic status of is disputed: it only appears in handful of reconstructible roots that themselves are often disputed. All of the reconstructed roots with */b/ inside are usually confined to a few Indo-European branches, likely representing late PIE dialectalism. Such */b/ is then usually explained away as a result of some typologically common dissimilatory process, or some rather recent phonological developments.

At best, PIE remains a highly marginal phoneme.

Coronals/dentals

The standard reconstruction identified three coronal/dental stops: . They are symbolically grouped with the cover symbol T.

In so-called "thorn clusters" of the form TK in all branches except Anatolian and Tocharian a metathesis occurred, resulting in dorsal-coronal clusters of non-obvious phonetic makeup. Metathetized and unmetathetized forms survive in different ablaut grades of the root } "burn" (whence also English day) in Sanskrit, "is being burnt" < } and "burns" < }. See the section on PIE phonological rules, below, for more discussion and examples.

Dorsals

According to the traditional reconstruction, such as the one laid out in Brugmann's Grundriss der vergleichenden Grammatik der indogermanischen Sprachen more than century ago, three series of velars are reconstructed for PIE:
  • "Palatovelars" (or simply "palatals"), (also transcribed or or .
  • "Plain velars" (or "pure velars"), .
  • Labiovelars, (also transcribed ). Raised stands for labialisation
    Labialisation
    Labialization is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally restricted to consonants. When vowels involve the lips, they are called rounded.The most common...

     (lip-rounding) accompanying the articulation of velar sounds.


The terms "palatovelar" and "plain velar" are in quotes because they are traditional terms but are unlikely to represent the actual pronunciation of these sounds in PIE. The current consensus is that the "palatovelars" were in fact simple velars, i.e. , while the "plain velars" were pronounced farther back, perhaps actually uvular consonant
Uvular consonant
Uvulars are consonants articulated with the back of the tongue against or near the uvula, that is, further back in the mouth than velar consonants. Uvulars may be plosives, fricatives, nasal stops, trills, or approximants, though the IPA does not provide a separate symbol for the approximant, and...

s, i.e. . Meanwhile, the labiovelars were exactly like the "plain velars" but labialized, i.e. in the current understanding. These conclusions are suggested by the following evidence:
  • The "palatovelar" series was the most common; meanwhile the "plain velar" was by far the least common, and never occurred in any affixes. In known languages with multiple velar series, the normal velar series is usually the most common.
  • There is no evidence whatsoever of there ever having been any palatalization
    Palatalization
    In linguistics, palatalization , also palatization, may refer to two different processes by which a sound, usually a consonant, comes to be produced with the tongue in a position in the mouth near the palate....

     in the early history of the velars in any of the Centum branches. If the "palatovelars" were in fact palatalized in PIE, there would have had to be a single, very early, uniform depalatalization in all (and only) the Centum branches — depalatalization is cross-linguistically far less likely than palatalization, and hence unlikely to have occurred separately in each Centum branch, and if such a situation did occur, it almost certainly would have left evidence of prior palatalization in some of the branches. However, there is no evidence at all that the Centum branches ever formed a clade
    Clade
    A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

     (i.e. possess a common ancestor that is later than PIE as a whole, in which the putative depalatalization would have occurred). Quite the contrary, some evidence indicates that Anatolian
    Anatolian languages
    The Anatolian languages comprise a group of extinct Indo-European languages that were spoken in Asia Minor, the best attested of them being the Hittite language.-Origins:...

     and Tocharian, two of the Centum branches (note, however, that the Luwian branch of Anatolian seems to have satem reflexes, and Melchert reconstructs the palatal stops traditionally assumed for PIE for Proto-Anatolian, as well), were the first and second branches, respectively, to have split off from PIE, but this is uncertain and disputed.

Satem and Centum languages

The Satem group of languages merged the labiovelars with the plain velar series , and the palatovelars became fricatives or affricates. In some phonological conditions depalatalization occurred, yielding what appears to be a Centum reflex in a Satem language.

The Centum group of languages on the other hand merged the palatovelars with the plain velar series .
The problem of three velar series

The existence of all three dorsal rows (series) has been disputed since the beginning of Indo-European studies. Today, most PIE linguists believe that all three series were distinct by the time of Late Proto-Indo-European, although a minority believe that the distinction between plain velar and palatovelar consonants was a later development of certain Satem languages; this belief was originally articuled by Antoine Meillet
Antoine Meillet
Paul Jules Antoine Meillet was one of the most important French linguists of the early 20th century. Meillet began his studies at the Sorbonne, where he was influenced by Michel Bréal, Ferdinand de Saussure, and the members of the Année Sociologique. In 1890 he was part of a research trip to the...

 in 1894 and argued more recently by Frederik Kortlandt
Frederik Kortlandt
Frederik Herman Henri Kortlandt is a professor of descriptive and comparative linguistics at Leiden University in the Netherlands. He is an expert on Baltic and Slavic languages, the Indo-European languages in general, and Proto-Indo-European, though he has also published studies of languages in...

 and others. This argument contends that PIE had only two series, a simple velar and a labiovelar. The Satem languages palatalized the plain velar series in most positions, but the plain velars remained in some environments. These environments are typically reconstructed as before or after /u/, after /s/, and before /r/ or /a/; also apparently before /m/ and /n/ in some Baltic dialects. (This is conceptually similar to the change in Proto-Germanic whereby e.g. /t/ became /θ/ most of the time, but remained as /t/ after original /s/, /k/ or /p/.) The original allophonic distinction was disturbed when the labiovelars were merged with the plain velars. This produced a new phonemic distinction between palatal and plain velars, with an unpredictable alternation between palatal and plain in related forms of some roots (those from original plain velars) but not others (those from original labiovelars). Subsequent analogical processes generalized either the plain or palatal consonant in all forms of a particular root. Those roots where the plain consonant was generalized are those traditionally reconstructed as having "plain velars" in the parent language, in contrast to "palatovelars".

The basic arguments in favor of two velar series are:
  • The plain velar series is statistically rarer than the other two, is almost entirely absent from affixes, and appears most often in certain phonological environments (described above).
  • Alternations between plain velars and palatals are common in a number of roots across different Satem languages, where the same root appears with a palatal in some languages but a plain velar in others (most commonly Baltic or Slavic; occasionally Armenian, but rarely or never the Indo-Iranian languages). This is consistent with the analogical generalization of one or another consonant in an originally alternating paradigm, but difficult to explain otherwise.
  • The above explanation suggests that in Late PIE times the Satem languages were in close contact with each other. This is confirmed by independent evidence: The geographical closeness of current Satem languages and certain other shared innovations (the Ruki sound law
    Ruki sound law
    Ruki refers to a sound change in Balto-Slavic, Albanian, Armenian, and Indo-Iranian, wherein an original phoneme changed into after the consonants , , and the semi-vowels , , or:...

     and early palatalization of velars before front vowels).
  • The traditional explanation of a three-way dorsal split requires that all Centum languages share a common innovation that eliminated the palatovelar series. Unlike for the Satem languages, however, there is no evidence of any areal connection among the Centum languages, and in fact there is evidence against such a connection—the Centum languages are geographically noncontiguous. Furthermore, if such an areal innovation happened, we would expect to see some dialect differences in its implementation (cf. the above differences between Balto-Slavic and Indo-Iranian), and residual evidence of a distinct palatalized series (such evidence for a distinct labiovelar series does exist in the Satem languages; see below). In fact, however, neither type of evidence exists, suggesting that there was never a palatovelar series in the Centum languages.


The basic arguments in favor of three velar series are:
  • Many instances of plain velars occur in roots that have no evidence of any of the putative environments that trigger plain velars, and no obvious mechanism for the plain velar to have come in contact with any such environment; as a result, the comparative method
    Comparative method
    In linguistics, the comparative method is a technique for studying the development of languages by performing a feature-by-feature comparison of two or more languages with common descent from a shared ancestor, as opposed to the method of internal reconstruction, which analyzes the internal...

     requires us to reconstruct three series.
  • Evidence from the Anatolian language Luwian
    Luwian language
    Luwian is an extinct language of the Anatolian branch of the Indo-European language family. Luwian is closely related to Hittite, and was among the languages spoken during the second and first millennia BC by population groups in central and western Anatolia and northern Syria...

     attests a three-way velar distinction > z (probably [ts]); > k; > ku (probably [kʷ]). There is no evidence of any connection between Luwian and any Satem language (labiovelars are still preserved, Ruki sound law
    Ruki sound law
    Ruki refers to a sound change in Balto-Slavic, Albanian, Armenian, and Indo-Iranian, wherein an original phoneme changed into after the consonants , , and the semi-vowels , , or:...

     is absent), and the Anatolian branch split off very early from PIE. Hence, the three-way distinction must be reconstructed for the parent language. (This is a strong argument in favor of the traditional three-way system; in response, proponents of the two-way system have attacked the underlying evidence, claiming that it "hinges upon especially difficult or vague or otherwise dubious etymologies" (e.g. Sihler 1995).) Melchert originally claimed that the change > z was unconditional, and subsequently revised the assertion to a conditional change occurring only before front vowels, /y/, or /w/; however, this does not fundamentally alter the situation, as plain-velar apparently remains as such in the same context. Melchert also asserts that, contrary to Sihler, the etymological distinction between and in the relevant positions is well-established.
  • According to Ringe (2006), there are root constraints that prevent the occurrence of a "palatovelar" and labiovelar, or two "plain velars", in the same root; but these do not apply to roots containing, e.g. a palatovelar and plain velar.


It should be noted that there is residual evidence of various sorts in the Satem languages of a former distinction between velar and labiovelar consonants:
  • In Sanskrit and Balto-Slavic, in some environments, resonant consonants (denoted by /R/) become /iR/ after plain velars but /uR/ after labiovelars.
  • In Armenian
    Armenian language
    The Armenian language is an Indo-European language spoken by the Armenian people. It is the official language of the Republic of Armenia as well as in the region of Nagorno-Karabakh. The language is also widely spoken by Armenian communities in the Armenian diaspora...

    , some linguists assert that /kʷ/ is distinguishable from /k/ before front vowels
  • Some linguists assert that in Albanian
    Albanian language
    Albanian is an Indo-European language spoken by approximately 7.6 million people, primarily in Albania and Kosovo but also in other areas of the Balkans in which there is an Albanian population, including western Macedonia, southern Montenegro, southern Serbia and northwestern Greece...

    , /kʷ/ and /gʷ/ are distinguishable from /k/ and /g/ before front vowels


This evidence shows that the labiovelar series was distinct from the plain velar series in PIE, and cannot have been a secondary development in the Centum languages; but it says nothing about the palatovelar vs. plain velar series.

In addition, modern proponents of the three-way distinction do not deny the final two points made in the arguments in favor of the two-way distinction, concerning the unity of the Satem group and lack of such unity in the Centum group. Rather, they claim that the Centum change did indeed occur independently in multiple Centum subgroups (at the very least, Tocharian, Anatolian and Western IE), but was a phonologically natural change given the current interpretation of the "palatovelar" series as plain-velar and the "plain velar" series as back-velar or uvular, and given the minimal functional load of the plain-velar/palatovelar distinction. Since there was never any palatalization in the IE dialects leading to the Centum languages, there is no reason to expect any palatal residues; furthermore, it is phonologically entirely natural that a former plain-velar vs. back-velar/uvular distinction would leave no distinctive residues on adjacent segments.

It is quite possible to use the traditional three-way distinction while remaining agnostic on the issue of whether it represents the actual state of the parent language or is an artifact of later developments in the Satem branch. It is also quite possible to take a compromise position asserting that the three-way distinction did indeed exist in late PIE but simultaneously did in fact develop from an earlier two-way distinction through the same mechanism and in the same environments traditionally claimed to have triggered the plain/palatal distinction in the Satem languages.

Fricatives

The only certain PIE fricative phoneme was a strident sound, whose phonetic realization could possibly range from [s] to palatalized [ɕ] or [ʃ
Voiceless postalveolar fricative
The voiceless palato-alveolar fricative or voiceless domed postalveolar fricative is a type of consonantal sound, used in many spoken languages, including English...

]. It had a voiced allophone *z that emerged by assimilation in words such as *nisdós 'nest', and which later became phonemicized in some daughter languages. Some PIE roots have variants with *s appearing initially: such *s is called s-mobile.

The "laryngeals" may have been fricatives, but there is no consensus as to their phonetic realization.

Laryngeals

The phonemes , with cover symbol also denoting "unknown laryngeal" (or and ), stand for three "laryngeal
Laryngeal theory
The laryngeal theory is a generally accepted theory of historical linguistics which proposes the existence of one, or a set of three , consonant sounds termed "laryngeals" that appear in most current reconstructions of the Proto-Indo-European language...

" phonemes. One should note that the term laryngeal as a phonetic description is out of date, retained only because its usage has become standard in the field.

Phonetic value of the laryngeal phonemes is disputable; various suggestions for their exact phonetic value have been made, ranging from cautious claims that all that can be said with certainty is that represented a velar fricative pronounced far back in the mouth, and that exhibited lip-rounding up to more definite proposal; e.g. Meier-Brügger
Michael Meier-Brügger
Michael Meier-Brügger is a Swiss linguist and Indo-Europeanist.He has been professor of comparative and Indo-European linguistics at the Free University of Berlin since 1996. After receiving his Ph.D. from the University of Zurich in 1973, he became Ernst Risch's assistant there before going on to...

 writes that realizations of = [h], = [χ] and = [ɣ] or [ɣʷ] "are in all probability accurate". Other commonly cited speculations for are ʔ ʕ ʕʷ (e.g. Beekes) and x χ~ħ xʷ. It is sometimes claimed that may have been two consonants, ʔ and h, that fell together. A consensus seems to be emerging, however, that is unlikely to have been a glottal stop /ʔ/, as all three laryngeals pattern similarly to each other and to fricatives in other languages (and similarly to PIE /s/, the only other fricative). It is possible, however, that all three laryngeals ultimately fell together as a glottal stop in some languages. Evidence for this development in Balto-Slavic comes from the eventual development of post-vocalic laryngeals into a register distinction commonly described as "acute" (vs. "circumflex" register on long vocalics not originally closed by a laryngeal) and marked in some fashion on all long syllables, whether stressed or not; furthermore, in some circumstances original acute register is reflected by a "broken tone" (i.e. glottalized vowel) in modern Latvian
Latvian language
Latvian is the official state language of Latvia. It is also sometimes referred to as Lettish. There are about 1.4 million native Latvian speakers in Latvia and about 150,000 abroad. The Latvian language has a relatively large number of non-native speakers, atypical for a small language...

.

The schwa indogermanicum symbol ə is commonly used for a laryngeal between consonants, in a "syllabic" position.

Glottalic theory

The phonetical values of the three stop series are traditionally reconstructed as voiceless (e.g. */t/), voiced (e.g. */d/) and voiced aspirated (e.g. */dʰ/). However, this system is not found in any descendant language (Sanskrit
Sanskrit
Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 still has all three, but has added a fourth series of voiceless aspirated, e.g. /tʰ/), and is vanishingly rare in any recorded languages. The rarity of */b/ is also unusual. Additionally, PIE roots
Proto-Indo-European root
The roots of the reconstructed Proto-Indo-European language are basic parts of words that carry a lexical meaning, so-called morphemes. PIE roots always have verbal meaning like "to eat" or "to run", as opposed to nouns , adjectives , or other parts of speech. Roots never occur alone in the language...

 have a constraint which prohibits roots mixing voiceless and voiced aspirate stops, as well as roots containing two voiced stops. These facts have led some scholars to reassess this part of the reconstruction, replacing the voiced stops by glottalized
Glottalization
Glottalization is the complete or partial closure of the glottis during the articulation of another sound. Glottalization of vowels and other sonorants is most often realized as creaky voice...

and the voiced aspirated stops by plain voiced. Direct evidence for glottalization is limited, but there is some indirect evidence, including Winter's law in Balto-Slavic.

Sonorants

In a phonological sense, sonorants in Proto-Indo-European were those segments that could stay in syllable nucleus (i.e. they could be syllabic) and out of it (i.e. they could be non-syllablic). PIE sonorants are the liquids, nasals and glides: , all grouped with the cover symbol R.

All of them had allophones in a syllabic position, which is generally between consonants, word-initially before consonants and word-finally after a consonant. They are marked as: . One should note that, even though *i and *u were phonetically certainly vowels, phonologically they were non-syllabic sonorants.

Reflexes

  • Proto-Celtic, Albanian
    Albanian language
    Albanian is an Indo-European language spoken by approximately 7.6 million people, primarily in Albania and Kosovo but also in other areas of the Balkans in which there is an Albanian population, including western Macedonia, southern Montenegro, southern Serbia and northwestern Greece...

    , Proto-Balto-Slavic and Proto-Iranian
    Iranian languages
    The Iranian languages form a subfamily of the Indo-Iranian languages which in turn is a subgroup of Indo-European language family. They have been and are spoken by Iranian peoples....

     merged the voiced aspirated series with the plain voiced series . (In Proto-Balto-Slavic this postdated Winter's law. Proto-Celtic retains the distinction between - the former became */gw/ while the latter became */b/.)
  • Proto-Germanic
    Proto-Germanic language
    Proto-Germanic , or Common Germanic, as it is sometimes known, is the unattested, reconstructed proto-language of all the Germanic languages, such as modern English, Frisian, Dutch, Afrikaans, German, Luxembourgish, Danish, Norwegian, Icelandic, Faroese, and Swedish.The Proto-Germanic language is...

     underwent Grimm's law
    Grimm's law
    Grimm's law , named for Jacob Grimm, is a set of statements describing the inherited Proto-Indo-European stops as they developed in Proto-Germanic in the 1st millennium BC...

    , changing voiceless stops into fricatives, devoicing unaspirated voiced stops, and de-aspirating voiced aspirates.
  • Grassmann's law
    Grassmann's Law
    Grassmann's law, named after its discoverer Hermann Grassmann, is a dissimilatory phonological process in Ancient Greek and Sanskrit which states that if an aspirated consonant is followed by another aspirated consonant in the next syllable, the first one loses the aspiration...

     ( > , e.g. > ) and Bartholomae's law
    Bartholomae's law
    Bartholomae's law is an early Indo-European sound law affecting the Indo-Iranian family. It states that in a cluster of two or more obstruents , any one of which is a voiced aspirated stop anywhere in the sequence, the whole cluster becomes voiced and aspirated...

     ( > , e.g. > ) describe the behaviour of aspirates in particular contexts in some early daughter languages.


Sanskrit, Greek, and Germanic, along with Latin to some extent, are the most important for reconstructing PIE consonants, as all of these languages keep the three series of stops (voiceless, voiced and voiced-aspirated) separate. In Germanic, Verner's law
Verner's law
Verner's law, stated by Karl Verner in 1875, describes a historical sound change in the Proto-Germanic language whereby voiceless fricatives *f, *þ, *s, *h, *hʷ, when immediately following an unstressed syllable in the same word, underwent voicing and became respectively the fricatives *b, *d, *z,...

 and changes to labiovelars (especially outside of Gothic
Gothic language
Gothic is an extinct Germanic language that was spoken by the Goths. It is known primarily from the Codex Argenteus, a 6th-century copy of a 4th-century Bible translation, and is the only East Germanic language with a sizable Text corpus...

) obscure some of the original distinctions; but on the other hand, Germanic is not subject to the assimilations of Grassmann's law
Grassmann's Law
Grassmann's law, named after its discoverer Hermann Grassmann, is a dissimilatory phonological process in Ancient Greek and Sanskrit which states that if an aspirated consonant is followed by another aspirated consonant in the next syllable, the first one loses the aspiration...

, which affects both Greek and Sanskrit. Latin also keeps the three series separate, but largely obscures the distinctions among voiced-aspirated consonants in initial position (all except /gh/ become /f/) and collapses many distinctions in medial position. Greek is especially important for reconstructing labiovelars, as other languages tend to delabialize them in many positions.

Anatolian and Greek are the most important languages for reconstructing the laryngeals. Anatolian directly preserves many laryngeals, while Greek preserves traces of laryngeals in positions (e.g. at the beginning of a word) where they disappear in many other languages, and reflects each laryngeal different from the others (the so-called triple reflex) in most contexts. Balto-Slavic languages
Balto-Slavic languages
The Balto-Slavic language group traditionally comprises Baltic and Slavic languages, belonging to the Indo-European family of languages. Baltic and Slavic languages share several linguistic traits not found in any other Indo-European branch, which points to the period of common development...

 are sometimes important in reconstructing laryngeals, since they are fairly directly represented in the distinction between "acute" and "circumflex" vowels. Old Avestan faithfully preserves numerous relics (e.g. laryngeal hiatus, laryngeal aspiration, laryngeal lengthening) triggered by ablaut alternations in laryngeal-stem nouns, but the paucity of the Old Avestan corpus prevents it from being more useful. Vedic Sanskrit
Vedic Sanskrit
Vedic Sanskrit is an old Indo-Aryan language. It is an archaic form of Sanskrit, an early descendant of Proto-Indo-Iranian. It is closely related to Avestan, the oldest preserved Iranian language...

 preserves the same relics rather less faithfully, but in greater quantity, making it sometimes useful.

Introduction

In a strict phonological sense, Proto-Indo-European had only two vowels: */e/ and */o/. Their phonetic and phonological interpretation is uncertain: it was most probably the opposition of high and low vowel, rather than the opposition of back and front vowel. Typologically, such system is similar to the vowel systems of Northwest Caucasian languages
Northwest Caucasian languages
The Northwest Caucasian languages, also called Abkhazo-Adyghean, or sometimes Pontic as opposed to Caspian for the Northeast Caucasian languages, are a group of languages spoken in the Caucasus region, chiefly in Russia , the disputed territory of Abkhazia, and Turkey, with smaller communities...

, such as Abkhaz:
PIE vowel segments,
System I
high/close *o [ə]
low/open *e [a]


However, these two vowels were not the only syllabic segments in PIE; beside *e and *o, syllable nucleus could also contain sonorants *y, *w, *m, *n, *l, and *r. Such syllabic sonorants therefore surface as vowels, but are distinguished from the real vowels by the fact that they participate in ablaut
Indo-European ablaut
In linguistics, ablaut is a system of apophony in Proto-Indo-European and its far-reaching consequences in all of the modern Indo-European languages...

 alternations with their nonsyllabic pairs, and by the distribution; while the vowels *e and *o can be positioned only in the syllable nucleus, sonorants can also make appearance in both syllable coda and onset.

Also, sonorants can comprise the second part of a complex syllable nucleus, i.e. they can form diphthongs with real vowels: *ey, *oy, *ew, *ow, *em, *en, *om, *on etc. From a phonetical viewpoint, syllabic allophones of sonorants *y and *w act like vowels, and are marked as vocalics *i and *u.

Depending on whether one considers segments *i and *u vowels (according to the phonetical criterion), or sonorants (according to the distributional criterion), Proto-Indo-European vowel system would comprise 2 or 4 members. The system of four (short) vowels - *e, *o, *i, *u - can then be presented as oppositions of high (*i, *u) : low (*e, *o) and front (*e, *i) : back (*o, *u):
PIE vowel segments,
System II
Front
Front vowel
A front vowel is a type of vowel sound used in some spoken languages. The defining characteristic of a front vowel is that the tongue is positioned as far in front as possible in the mouth without creating a constriction that would be classified as a consonant. Front vowels are sometimes also...

Back
Back vowel
A back vowel is a type of vowel sound used in spoken languages. The defining characteristic of a back vowel is that the tongue is positioned as far back as possible in the mouth without creating a constriction that would be classified as a consonant. Back vowels are sometimes also called dark...

High *i *u
Low *e *o


In certain morphological (e.g., as a result of Proto-Indo-European ablaut
Indo-European ablaut
In linguistics, ablaut is a system of apophony in Proto-Indo-European and its far-reaching consequences in all of the modern Indo-European languages...

) and phonological conditions (e.g. in the last syllable of nominative singular of a noun ending on sonorant, in root syllable in sigmatic aorist etc.; cf. Szemerényi's law
Szemerényi's law
Szemerényi's law is a Proto-Indo-European phonological rule, named after Hungarian linguist Oswald Szemerényi, according to which word-final clusters of vowels , resonants and of either */s/ or */h₂/ are simplified by dropping the word-final fricative , with compensatory lengthening of the...

, Stang's law
Stang's law
Stang's law is a Proto-Indo-European phonological rule named after Norwegian linguist Christian Stang. The law governs the word-final sequences of a vowel, followed by a laryngeal or a semivowel */y/ or */w/, followed by a nasal, and according to the law those sequences are simplified in a way that...

) vowels *e and *o would lengthen, yielding respective lengthened-grade variants. Basic, lexical forms of words in PIE contain therefore only short vowels; on the basis of well-established morphophonological rules forms with long vowels *ē and *ō appear.

Lengthening of vowels must have been a phonologically conditioned change in Early Proto-Indo-European, but at the period just before the dissolution of Proto-Indo-European speaking community, which is usually reconstructed, it is not possible to phonologically predict the appearance of all long vowels, because the phonologically justified resulting long vowels have begun to spread analogically to other forms in which they were not phonologically justified. Hence, the prosodically long */e/ in 'father' results by the application of Szemerényi's law
Szemerényi's law
Szemerényi's law is a Proto-Indo-European phonological rule, named after Hungarian linguist Oswald Szemerényi, according to which word-final clusters of vowels , resonants and of either */s/ or */h₂/ are simplified by dropping the word-final fricative , with compensatory lengthening of the...

, a synchronic phonological rule that operated within the PIE, but prosodically long */o/ in 'foot' is analogically leveled.

It is possible that Proto-Indo-European had a few morphologically isolated words that contained the vowel *a, e.g. *dap- 'sacrifice' (Latin daps, Ancient Greek dapánē, Old Irish dúas); or appearing as a first part of a diphthong *ay, e.g. *laywos 'left' (Latin laevus, Ancient Greek laiós, OCS lěvъ). The phonemic status of *a has been fiercely disputed; for example Beekes expressly concludes: There are thus no grounds for PIE phoneme *a, and the same conclusion is reached by his former student Alexander Lubotsky. After the discovery of Hittite
Hittite language
Hittite is the extinct language once spoken by the Hittites, a people who created an empire centred on Hattusa in north-central Anatolia...

 and the advent of laryngeal theory
Laryngeal theory
The laryngeal theory is a generally accepted theory of historical linguistics which proposes the existence of one, or a set of three , consonant sounds termed "laryngeals" that appear in most current reconstructions of the Proto-Indo-European language...

, basically every instance of previous *a could be reduced to *eh₂. Against the possibility of PIE phoneme *a, that is even today held by some Indo-Europeanists, the following can be said: vowel *a does not participate in ablaut alternations (i.e. it does not alternate with other vowels, as the "real" PIE vowels *e, *o, *ē, *ō do), it makes no appearance in suffixes and endings, it appears in very confined set of positions (usually after initial *k) and the reflexes of words upon which *a is reconstructed are usually confined only to a few Indo-European languages which makes it possible to ascribe it to some late PIE dialectalism, or are of expressive character thus not being suitable for comparative analysis, or are argued to have been borrowed from some other language which had phonemic *a (e.g. Proto-Semitic *θawru > PIE *táwros 'bull, steer').

However, others, like Mayrhofer
Manfred Mayrhofer
Manfred Mayrhofer is an Indo-Europeanist specialized on Indo-Iranian languages. Mayrhofer is professor emeritus at the University of Vienna. He is noted for his etymological dictionary of Sanskrit....

, argue that PIE did in fact have and phonemes independent of .

In fact, Stang's law, a phonological rule active at the Proto-Indo-European stage, produces from older as the ending of the accusative singular of the stems in (and possibly from older in the respective accusative plural), and therefore compels the assumption that the phoneme was present in the language.

Taking into account the long vowels, as well as the (marginal) vowel *a, the following maximal system of PIE vocalism emerges:
PIE vowel segments,
System III
*i *u
*e *o
(*a)

Reflexes

Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 reflects the original PIE vowel system most faithfully, with few changes to PIE vowels in any syllable; however, loss of certain consonants, especially /s/, /w/ and /y/, often triggers compensatory lengthening
Compensatory lengthening
Compensatory lengthening in phonology and historical linguistics is the lengthening of a vowel sound that happens upon the loss of a following consonant, usually in the syllable coda...

 or contraction of vowels in hiatus
Hiatus (linguistics)
In phonology, hiatus or diaeresis refers to two vowel sounds occurring in adjacent syllables, with no intervening consonant. When two adjacent vowel sounds occur in the same syllable, the result is instead described as a diphthong....

, which can complicate reconstruction.

Sanskrit
Sanskrit
Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 and Avestan merge /e/, /a/ and /o/ into a single vowel /a/ (with a corresponding merger in the long vowels), but reflect PIE length differences (especially due to ablaut) even more faithfully than Greek, and do not have the same issues with consonant loss that Greek does. Furthermore, /o/ can often be reconstructed through Brugmann's law
Brugmann's law
Brugmann's law, named for Karl Brugmann, states that Proto-Indo-European in non-final syllables became *ā in open syllables in Indo-Iranian. Everywhere else the outcome was *ǎ, the same as the reflexes of PIE *e and *a...

, and /e/ through the "law of palatals" (see Proto-Indo-Iranian language
Proto-Indo-Iranian language
Proto-Indo-Iranian is the reconstructed proto-language of the Indo-Iranian branch of Indo-European. Its speakers, the hypothetical Proto-Indo-Iranians, are assumed to have lived in the late 3rd millennium BC, and are usually connected with the early Andronovo archaeological...

).

Germanic languages
Germanic languages
The Germanic languages constitute a sub-branch of the Indo-European language family. The common ancestor of all of the languages in this branch is called Proto-Germanic , which was spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...

 show merger of long and short /a/ and /o/, but (especially in the case of Gothic
Gothic language
Gothic is an extinct Germanic language that was spoken by the Goths. It is known primarily from the Codex Argenteus, a 6th-century copy of a 4th-century Bible translation, and is the only East Germanic language with a sizable Text corpus...

) are still important for reconstructing PIE vowels. Balto-Slavic languages
Balto-Slavic languages
The Balto-Slavic language group traditionally comprises Baltic and Slavic languages, belonging to the Indo-European family of languages. Baltic and Slavic languages share several linguistic traits not found in any other Indo-European branch, which points to the period of common development...

 are similar, again showing merger of short /a/ and /o/ (and for Slavic languages, also long /a/ and /o/).

Evidence from Anatolian
Anatolian languages
The Anatolian languages comprise a group of extinct Indo-European languages that were spoken in Asia Minor, the best attested of them being the Hittite language.-Origins:...

 and Tocharian can be important due to the archaism of these languages, but is often difficult to interpret; Tocharian, especially, has complex and far-reaching vowel innovations.

Italic languages
Italic languages
The Italic subfamily is a member of the Indo-European language family. It includes the Romance languages derived from Latin , and a number of extinct languages of the Italian Peninsula, including Umbrian, Oscan, Faliscan, and Latin.In the past various definitions of "Italic" have prevailed...

 and Celtic languages
Celtic languages
The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 do not unilaterally merge any vowels, but have such far-reaching vowel changes (especially in the case of the Celtic languages
Celtic languages
The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

) that they are somewhat less useful for PIE. Albanian
Albanian language
Albanian is an Indo-European language spoken by approximately 7.6 million people, primarily in Albania and Kosovo but also in other areas of the Balkans in which there is an Albanian population, including western Macedonia, southern Montenegro, southern Serbia and northwestern Greece...

 and Armenian
Armenian language
The Armenian language is an Indo-European language spoken by the Armenian people. It is the official language of the Republic of Armenia as well as in the region of Nagorno-Karabakh. The language is also widely spoken by Armenian communities in the Armenian diaspora...

 are least useful, as they are attested relatively late, have borrowed heavily from other languages, and have complex and ill-understood vowel changes.

Accent

PIE had a free pitch accent
Pitch accent
Pitch accent is a linguistic term of convenience for a variety of restricted tone systems that use variations in pitch to give prominence to a syllable or mora within a word. The placement of this tone or the way it is realized can give different meanings to otherwise similar words...

, which could appear on any syllable and whose position often varied among different members of a paradigm (e.g. between singular and plural of a verbal paradigm, or between nominative/accusative and oblique cases of a nominal paradigm). The location of the pitch accent is closely associated with ablaut
Indo-European ablaut
In linguistics, ablaut is a system of apophony in Proto-Indo-European and its far-reaching consequences in all of the modern Indo-European languages...

 variations, especially between normal-grade vowels (/e/ and /o/) and zero-grade vowels (i.e. lack of a vowel).

Generally, thematic nouns and verbs (those with a "thematic vowel" between root and ending, usually /e/ or /o/) had a fixed accent, which (depending on the particular noun or verb) could be either on the root or the ending. These words also had no ablaut variations within their paradigms. (However, accent and ablaut were still associated; for example, thematic verbs with root accent tended to have e-grade ablaut in the root, while those ending accent tended to have zero-grade ablaut in the root.) On the other hand, athematic nouns and verbs usually had mobile accent, with varied between strong forms, with root accent and full grade in the root (e.g. the singular active of verbs, and the nominative and accusative of nouns), and weak forms, with ending accent and zero grade in the root (e.g. the plural active and all forms of the middle of verbs, and the oblique cases of nouns). Some nouns and verbs, on the other hand, had a different pattern, with ablaut variation between lengthened and full grade and mostly fixed accent on the root; these are termed Narten stems. Additional patterns exist for both nouns and verbs. For example, some nouns (so-called acrostatic nouns, one of the oldest classes of noun) has fixed accent on the root, with ablaut variation between o-grade and e-grade, while hysterodynamic nouns have zero-grade root with a mobile accent that varies between suffix and ending, with corresponding ablaut variations in the suffix.

The accent is best preserved in Vedic Sanskrit
Vedic Sanskrit
Vedic Sanskrit is an old Indo-Aryan language. It is an archaic form of Sanskrit, an early descendant of Proto-Indo-Iranian. It is closely related to Avestan, the oldest preserved Iranian language...

 and (in the case of nouns) Ancient Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

. It is also reflected to some extent in the accentual patterns of the Balto-Slavic languages
Balto-Slavic languages
The Balto-Slavic language group traditionally comprises Baltic and Slavic languages, belonging to the Indo-European family of languages. Baltic and Slavic languages share several linguistic traits not found in any other Indo-European branch, which points to the period of common development...

 (e.g. Latvian
Latvian language
Latvian is the official state language of Latvia. It is also sometimes referred to as Lettish. There are about 1.4 million native Latvian speakers in Latvia and about 150,000 abroad. The Latvian language has a relatively large number of non-native speakers, atypical for a small language...

, Lithuanian
Lithuanian language
Lithuanian is the official state language of Lithuania and is recognized as one of the official languages of the European Union. There are about 2.96 million native Lithuanian speakers in Lithuania and about 170,000 abroad. Lithuanian is a Baltic language, closely related to Latvian, although they...

 and Serbo-Croatian
Serbo-Croatian language
Serbo-Croatian or Serbo-Croat, less commonly Bosnian/Croatian/Serbian , is a South Slavic language with multiple standards and the primary language of Serbia, Croatia, Bosnia and Herzegovina, and Montenegro...

). It is indirectly attested in a number of phenomena in other PIE languages, especially the Verner's law
Verner's law
Verner's law, stated by Karl Verner in 1875, describes a historical sound change in the Proto-Germanic language whereby voiceless fricatives *f, *þ, *s, *h, *hʷ, when immediately following an unstressed syllable in the same word, underwent voicing and became respectively the fricatives *b, *d, *z,...

 variations in the Germanic languages
Germanic languages
The Germanic languages constitute a sub-branch of the Indo-European language family. The common ancestor of all of the languages in this branch is called Proto-Germanic , which was spoken in approximately the mid-1st millennium BC in Iron Age northern Europe...

. In other languages (e.g. the Italic languages
Italic languages
The Italic subfamily is a member of the Indo-European language family. It includes the Romance languages derived from Latin , and a number of extinct languages of the Italian Peninsula, including Umbrian, Oscan, Faliscan, and Latin.In the past various definitions of "Italic" have prevailed...

 and Celtic languages
Celtic languages
The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

) it was lost without a trace. Other than in Modern Greek
Modern Greek
Modern Greek refers to the varieties of the Greek language spoken in the modern era. The beginning of the "modern" period of the language is often symbolically assigned to the fall of the Byzantine Empire in 1453, even though that date marks no clear linguistic boundary and many characteristic...

, the Balto-Slavic languages and (to some extent) Icelandic
Icelandic language
Icelandic is a North Germanic language, the main language of Iceland. Its closest relative is Faroese.Icelandic is an Indo-European language belonging to the North Germanic or Nordic branch of the Germanic languages. Historically, it was the westernmost of the Indo-European languages prior to the...

, few traces of the PIE accent remain in any modern languages.

Phonological rules

A number of phonological rules can be reconstructed for Proto-Indo-European. Some of them are disputed to be valid for "PIE proper", and are claimed to be later innovations in some of the daughter branches. Some of these laws are:
  1. Bartholomae's law
    Bartholomae's law
    Bartholomae's law is an early Indo-European sound law affecting the Indo-Iranian family. It states that in a cluster of two or more obstruents , any one of which is a voiced aspirated stop anywhere in the sequence, the whole cluster becomes voiced and aspirated...

    : >
    Passive participle of 'to learn, become aware of': } > } > (Grassmann's law
    Grassmann's Law
    Grassmann's law, named after its discoverer Hermann Grassmann, is a dissimilatory phonological process in Ancient Greek and Sanskrit which states that if an aspirated consonant is followed by another aspirated consonant in the next syllable, the first one loses the aspiration...

    ) Sanskrit buddhá.
    Law has been preserved in Indo-Iranian branch where it operates as a synchronic rule. There are some traces of it in Ancient Greek and Germanic, and possibly in Latin.
  2. TT > TsT: The cluster of two dental stops had dental fricative */s/ inserted between them.
    'eats' > > Hittite ezzi.
    This has been preserved in Hittite where cluster *tst is spelled as z .
  3. TK > KT > "Kþ" ("thorn clusters"): Dental stops that were placed behind PIE dorsals in the same syllable metathesized in all branches except in Tocharian and Anatolian (the earliest one that were to split from PIE matrix). Subsequent outcomes were varied.
    'bear' > > Latin ursus, Ancient Greek árktos, Sanskrit ṛ́kṣas but Hittite ḫartaggas /ḫartkas/ without metathesis.
    'decaying, decline, ruin' > > Ancient Greek phthísis, Sanskrit kṣítis, perhaps Latin sitis
  4. Siebs' law
    Siebs' law
    Siebs' law is a Proto-Indo-European phonological rule named after the German linguist Theodor Siebs. According to this law, if an s-mobile is added to a root that starts with a voiced or aspirated stop, that stop is devoiced.Compare:-Discussion:...

    : If s-mobile is added to the root that starts with voiced or aspirated stop, that stop is devoiced.
    > Latin fragor, but > > Sanskrit sphūrjati
  5. Stang's law
    Stang's law
    Stang's law is a Proto-Indo-European phonological rule named after Norwegian linguist Christian Stang. The law governs the word-final sequences of a vowel, followed by a laryngeal or a semivowel */y/ or */w/, followed by a nasal, and according to the law those sequences are simplified in a way that...

    : *Vwm > *Vːm; i.e. */w/ disappears and the preceding vowel lengthens in the last syllable behind word-final */m/. Some also add rules: *Vmm > *Vːm and } > *Vːm; and also *Vyi > *Vːy.
    *dyéwm 'sky' (accusative singular) > *dyḗm > Sanskrit dyā́m, acc. sg. of dyaús
    } 'cattle' (acc. sg.) > } > Sanskrit gā́m, acc. sg. of gaús
    accusative singular of } 'house' is , not .
  6. Szemerényi's law
    Szemerényi's law
    Szemerényi's law is a Proto-Indo-European phonological rule, named after Hungarian linguist Oswald Szemerényi, according to which word-final clusters of vowels , resonants and of either */s/ or */h₂/ are simplified by dropping the word-final fricative , with compensatory lengthening of the...

    : -VRs > VːR, > i.e. in word-final sequences of vowel, sonorant and */s/ or */h₂/ the fricative or laryngeal was dropped and the preceding vowel lengthened. This affected nominative singulars of numerous masculine and feminine nouns, as well as the nominoaccusative of neuter collectives.
    'father' > > Ancient Greek patḗr, Sanskrit pitā́
  7. Laryngeal deletion rules: See below.

Thorn clusters

A problem in the reconstruction of PIE concerns some cognate sets in which Indo-Iranian sibilants in clusters with dorsals exceptionally correspond to coronal stops in certain other branches. 'Bear' and 'decaying' above are examples; some others are Sanskrit tákṣan 'artisan' vs. Greek téktōn 'carpenter', and Sanskrit kṣā́ḥ vs. Greek khthon both 'earth'. As was the case with the laryngeal theory
Laryngeal theory
The laryngeal theory is a generally accepted theory of historical linguistics which proposes the existence of one, or a set of three , consonant sounds termed "laryngeals" that appear in most current reconstructions of the Proto-Indo-European language...

, these cognate sets were first noted prior to the connection of Anatolian and Tocharian to PIE, and early reconstructions posited a new series of consonants to explain these correspondences. Brugmann 1897's systematic explanation augmented the PIE consonant system with a series of interdentals
Interdental consonant
Interdental consonants are produced by placing the blade of the tongue against the upper incisors...

 (nowhere directly attested) appearing only in clusters with dorsals, *kþ *khþh *gð *ghðh. The use of the letter thorn
Thorn (letter)
Thorn or þorn , is a letter in the Old English, Old Norse, and Icelandic alphabets, as well as some dialects of Middle English. It was also used in medieval Scandinavia, but was later replaced with the digraph th. The letter originated from the rune in the Elder Fuþark, called thorn in the...

 led to the name "thorn cluster" for these groups.

Anatolian and Tocharian evidence suggests that the original form of the thorn clusters was in fact *TK: Hittite has tēkan, tagnās, dagān and Tocharian A tkaṃ, tkan- for case-forms of 'earth', so that the development outside Anatolian and Tocharian involved a metathesis. The conventional notations *þ *ðʰ for the second elements of these metathesised clusters are still found, and some, including Fortson, continue to hold to the view that interdental fricatives were involved at some stage of PIE.

An alternative interpretation (e.g. Vennemann 1989, Schindler 1991 (informally and unpublished)) identifies these segments as alveolar affricates [t͡s d͡z]. In this view, thorn clusters developed as TK > TsK > KTs and then variously in daughter languages; this has the advantage that the first change can be identified with the TT > TsT rule above, which is then broadened in application to affrication of dental stops before any stops. Melchert has interpreted the Cuneiform Luvian īnzagan- 'inhumation', probably [ind͡zgan], from } 'in the earth', as preserving the intermediate stage of this process.

Laryngeal deletion rules

Once the laryngeal theory was developed, and the rules for sound change of laryngeals worked out, it was clear that there were a number of exceptions to the rules, in particular with regard to "syllabic" laryngeals (former "schwa indogermanicum") that occurred in non-initial syllables. It was long suggested that such syllabic laryngeals were simply deleted in certain of the daughters; this is based especially on the PIE word dhugh₂tér- "daughter", which appears in a number of branches (e.g. Germanic, Balto-Slavic) with no vowel in place of expected /a/ for "syllabic" /h₂/ (cf. English "daughter", Gothic daúhtar). With a better understanding of the role of ablaut, however, and a clearer understanding of which roots did and did not have laryngeals in them, it became clear that this suggestion cannot be correct. In particular, there are some cases where syllabic laryngeals in medial syllables delete in most or all daughter languages, and other cases where they do not delete even in Germanic and/or Balto-Slavic.

This has led to the more recent idea that PIE had a number of synchronic "laryngeal deletion" rules, where syllabic larnygeals in certain contexts were deleted even in the protolanguage. In the case of dhugh₂tér-, for example, it appears that PIE had an alternation between a "strong" stem dhugh₂tér- and a "weak" stem dhugtr-, where a deletion rule eliminated the larnygeal in the latter context but not the former one. Forms in daughter languages with or without the larnygeal are due to analogical generalization of one or the other protoforms.

This is a new area, and as a result there is no consensus on the number and nature of the deletion rules. A wide variety of rules have been proposed; Ringe (2006) identifies the following three as the most likely candidates (where C=any consonant, V=any vowel, H=any larngeal, R=any resonant):
  1. A laryngeal in the sequence oRHC was dropped. Example: *tórmos "borehole" from *terh₁- "bore" (cf. Gk tórmos "socket", OE þearm "intestine"). This seems to have operated particularly in the thematic optative suffix -oy-h₁-, which was reduced to -oy- in most forms.
  2. A larnygeal in the sequence VCHy was dropped. Examples: *wérye- "say" (present tense) from *werh₁- (cf. Homeric Greek eírei "(he) says", not *eréei); h₂érye- "plow" (present tense) from h₂erh₃- "plow" (cf. Lith. ãria "(he) plows", not *ária).
  3. A larnygeal in the sequence CH-CC was dropped, where a syllable boundary follows the larnygeal (i.e. the following two consonants are capable of occurring at the start of a word, as in tr- but not rt-). An example is the weak stem dhugtr- given above, compared to the strong stem dhugh₂tér-.


It seems unlikely that this is a correct and complete description of the actual phonological rules underlying larnygeal deletion. These rules do not account for all the potential cases of laryngeal deletion (hence the many other rules that have been proposed); for example, the laryngeal in the desiderative suffixes -h₁s- and -h₁sy- appears to delete after an obstruent but not a resonant. In any case it is difficult to determine when a particular larnygeal loss is due to a protolanguage rule vs. an instance of later analogy. In addition, as synchronic phonological rules the set of above rules is more complex than what is expected from a cross-linguistic standpoint, suggesting that some of the rules may have already been "morphologized" (incorporated into the morphology of certain constructions, such as the o-grade noun-forming rule or the rule forming y-presents); the above-mentioned larnygeal deletion in the desiderative suffixes may be an example of such morphologization.

Phonetic correspondences in daughter languages

The correlations among the Indo-European languages are for the most part straightforward, but there are some complications with the velar consonants. The languages divide into two groups, known as the Centum languages and Satem languages, based on the respective words for "hundred" in representative languages of each group (Latin and Avestan
Avestan language
Avestan is an East Iranian language known only from its use as the language of Zoroastrian scripture, i.e. the Avesta, from which it derives its name...

, respectively). Each group merges the PIE "plain velars" with one of the other two series, the Centum group merging "palatals" and "plain velars" while the Satem group merges labiovelars and "plain velars", removing the labialization in the process. The Satem group furthermore converts the "palatal" series into sibilant-type sounds. The following table summarizes the outcomes in the various daughters:
PIE *ḱ *ǵʰ *kʷ *gʷ *gʷʰ
Celtic k g g kw, pWithin Celtic, the "p-Celtic" and "q-Celtic" branches have different reflexes of PIE *kʷ: *ekwos → ekwos, epos. The Brythonic and Lepontic languages are p-Celtic; Goidelic and Celtiberian are q-Celtic; while different dialects of Gaulish had different realizations. b gw
Italic k g g, hPIE *ǵʰ → Latin /h/ or /g/, depending on its position in the word, and → Osco-Umbrian *kh → /h/. kw, pPIE *kʷ and *gʷ developed differently in the two Italic subgroups: /kw/, /gw/ in Latin (*kwis → kwis), and /p/, /b/ in Osco-Umbrian (*kwis → pis). gw, v, b f, v
Venetic k g h kw ? ?
Hellenic k g kh p, t, kPIE *kʷ, *gʷ, *gʷʰ have three reflexes in Greek dialects such as Attic and Doric:
/t, d, th/ before /e, é, i/ (IE *kʷis → Greek tis)
/k, g, kh/ before /u/ (IE *wl.kʷos → Greek lukos)
/p, b, ph/ before /a, o/ (IE *sekʷ- → Greek hep-)

However, in Mycenaean Greek *kw remained, and in Aeolian it leveled to /p/.
b, d, g ph, kh, th
Albanian s,PIE *ḱ, *ǵ, *ǵʰ were originally reflected in Balkan languages as spirants /þ/ and /ð/, later in Albanian turning into /s/, /z/. (k) z, (g) z, (d) k, s g, z g, z
Illyrian s z z ? ? ?
Thracian s z z k, kh g, k g
Armenian s c dz kh k g
Phrygian kThe Phrygian evidence is limited and often ambiguous, so the issue of kentum vs. satem reflexes is not completely settled; but the prevailing opinion is that Phrygian shows kentum reflexes along with secondary palatalisation of /k/ to /ts/ and /g/ to /dz/ before front vowels, as in most Romance languages, see Phrygian language#Phonology. g g, k k b g
Germanic h k g ~ ɣProto-Germanic reflexes of Indo-European voiced stops had spirant allophones, retained in intervocalic position in Gothic
Gothic language
Gothic is an extinct Germanic language that was spoken by the Goths. It is known primarily from the Codex Argenteus, a 6th-century copy of a 4th-century Bible translation, and is the only East Germanic language with a sizable Text corpus...

.
hw kw gwThis is the reflex after nasals. The outcome in other positions is disputed and may vary according to phonetic environment. See the note in Grimm's law#In detail. ~ w
Slavic s z z k g g
Baltic š ž ž k g g
Indic çIndic languages, ç represents a palatalized š sound hpIE *ǵʰ → proto-Indo-Iranian *džjh → Indic /h/, Iranian /z/. k, č g gh
Iranian s z z k, č g g
Anatolian kIn Luvic languages, yielding ts, at least under most circumstances. gIn Luvic languages, usually becoming *y initially, lost intervocalically. g kw gwIn Luvic languages, yielding simple w. gw
Tocharian k k k k, kw k, kw k, kw




In p-Celtic, Osco-Umbrian, and Aeolian Greek, *kw → /p/. This may be due to contact, perhaps in the Balkan region in the second millennium BC. The same /p/ also occurs in Hittite in a few pronominal forms (pippid "something, someone", cf. Latin quisquid).

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