In English spelling, the three letter rule (or short word rule) is the observation that one- and two-letter words tend to be function words such as I, at, he, if, of, or, etc. As a consequence of the rule, "content words" tend to have at least three letters

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

. In particular, content words containing fewer than three phoneme

Phoneme

In a language or dialect, a phoneme is the smallest segmental unit of sound employed to form meaningful contrasts between utterances....

s may be augmented with letters which are phonetically redundant, such as ebb, add, egg, inn, bee, awe, buy, owe, etc.

Origin

Many content words would be homograph

Homograph

A homograph is a word or a group of words that share the same written form but have different meanings. When spoken, the meanings may be distinguished by different pronunciations, in which case the words are also heteronyms. Words with the same writing and pronunciation A homograph (from the ,...

s of common function words if not for the latter's "redundant" letters: e.g. be/bee, in/inn, I/eye, to/two.
Otto Jespersen

Otto Jespersen

Jens Otto Harry Jespersen or Otto Jespersen was a Danish linguist who specialized in the grammar of the English language.He was born in Randers in northern Jutland and attended Copenhagen University, earning degrees in English, French, and Latin...

, describing the phenomenon in 1909, suggested the short spelling was a marker of reduced stress. Content words always have at least one stressed syllable

Syllable

A syllable is a unit of organization for a sequence of speech sounds. For example, the word water is composed of two syllables: wa and ter. A syllable is typically made up of a syllable nucleus with optional initial and final margins .Syllables are often considered the phonological "building...

, whereas function words are often completely unstressed

Weak form and strong form

In the phonology of stress-timed languages, the weak form of a word is a form that may be used when the word has no stress, and which is phonemically distinct from the strong form, used when the word is stressed. The strong form serves as the citation form or the isolation form when a word is...

; shorter spellings help to reflect this. (Interjection

Interjection

In grammar, an interjection or exclamation is a word used to express an emotion or sentiment on the part of the speaker . Filled pauses such as uh, er, um are also considered interjections...

s such as ah, eh, lo, yo are always stressed. Punctuation

Punctuation

Punctuation marks are symbols that indicate the structure and organization of written language, as well as intonation and pauses to be observed when reading aloud.In written English, punctuation is vital to disambiguate the meaning of sentences...

 serves to isolate these elements.)

In Old English

Old English language

Old English or Anglo-Saxon is an early form of the English language that was spoken and written by the Anglo-Saxons and their descendants in parts of what are now England and southeastern Scotland between at least the mid-5th century and the mid-12th century...

, inflection

Inflection

In grammar, inflection or inflexion is the modification of a word to express different grammatical categories such as tense, grammatical mood, grammatical voice, aspect, person, number, gender and case...

s increased the length of most content words in any case. Through to the seventeenth century, before English spelling was firmly settled, short forms for some content words did occur, such as eg (egg), ey (eye), lo (low), etc. Conversely, poets such as John Milton

John Milton

John Milton was an English poet, polemicist, a scholarly man of letters, and a civil servant for the Commonwealth of England under Oliver Cromwell...

 alternated between short and long forms for function words, depending on whether they occurred on or off the meter. For instance:

So spake the false Arch-Angel, and infus'd

Bad influence into th' unwarie brest

Of his Associate; hee together calls,

Or several one by one, the Regent Powers,

Under him Regent, tells, as he was taught,

(Paradise Lost

Paradise Lost

Paradise Lost is an epic poem in blank verse by the 17th-century English poet John Milton. It was originally published in 1667 in ten books, with a total of over ten thousand individual lines of verse...

, Book 5, ll. 694-698)

Exceptions

While many function words have more than two letters (and, she, were, therefore, etc.), the exceptions to the rule are rather two-letter content words. Only a few of these occur commonly in most texts: the words go (which also has a functional usage in the idiom going to do something), ox and, especially in American texts, ax.

English grammar is relatively flexible about converting words

Conversion (linguistics)

In linguistics, conversion, also called zero derivation, is a kind of word formation; specifically, it is the creation of a word from an existing word without any change in form...

 of one class to another, allowing verbal uses such as to up the ante or nominal uses such as the ins and outs. The verb forms be, am, is and do can be considered exceptions when used as lexical verb

Lexical verb

In linguistics a lexical verb or full verb is a member of an open class of verbs that includes all verbs except auxiliary verbs. Lexical verbs typically express action, state, or other predicate meaning. In contrast, auxiliary verbs express grammatical meaning...

s, which are content words, though not when used as auxiliary verb

Auxiliary verb

In linguistics, an auxiliary verb is a verb that gives further semantic or syntactic information about a main or full verb. In English, the extra meaning provided by an auxiliary verb alters the basic meaning of the main verb to make it have one or more of the following functions: passive voice,...

s, which are function words.

Many recent loanword

Loanword

A loanword is a word borrowed from a donor language and incorporated into a recipient language. By contrast, a calque or loan translation is a related concept where the meaning or idiom is borrowed rather than the lexical item itself. The word loanword is itself a calque of the German Lehnwort,...

s retain spelling from the source language or are romanized

Romanization

In linguistics, romanization or latinization is the representation of a written word or spoken speech with the Roman script, or a system for doing so, where the original word or language uses a different writing system . Methods of romanization include transliteration, for representing written...

 according to non-English phonetic conventions. This has resulted in short words such as the notes of the solfege

Solfege

In music, solfège is a pedagogical solmization technique for the teaching of sight-singing in which each note of the score is sung to a special syllable, called a solfège syllable...

 scale (do, re, mi, etc.; from 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...

 via Italian

Italian language

Italian is a Romance language spoken mainly in Europe: Italy, Switzerland, San Marino, Vatican City, by minorities in Malta, Monaco, Croatia, Slovenia, France, Libya, Eritrea, and Somalia, and by immigrant communities in the Americas and Australia...

) or the Greek alphabet

Greek alphabet

The Greek alphabet is the script that has been used to write the Greek language since at least 730 BC . The alphabet in its classical and modern form consists of 24 letters ordered in sequence from alpha to omega...

 (pi, nu, etc.) and miscellaneous others such as bo

Bo (weapon)

A bō or kon , is a long staff weapon used in Okinawa and feudal Japan. Bō are typically around long and are now used in Japanese martial arts, in particular bōjutsu...

, qi

Qi

In traditional Chinese culture, qì is an active principle forming part of any living thing. Qi is frequently translated as life energy, lifeforce, or energy flow. Qi is the central underlying principle in traditional Chinese medicine and martial arts...

, ka. Carney calls such words "exceptions which prove the rule

Exception that proves the rule

"The exception [that] proves the rule" is a frequently confused English idiom. The original meaning of this idiom is that the presence of an exception applying to a specific case establishes that a general rule existed....

, clearly marked as exotic by the spelling".

In less formal contexts, increasing numbers of abbreviation

Abbreviation

An abbreviation is a shortened form of a word or phrase. Usually, but not always, it consists of a letter or group of letters taken from the word or phrase...

s are found; short examples include ad(vertisement), bi(sexual). Similarly there are many nickname

Nickname

A nickname is "a usually familiar or humorous but sometimes pointed or cruel name given to a person or place, as a supposedly appropriate replacement for or addition to the proper name.", or a name similar in origin and pronunciation from the original name....

s; for example, Al, Ed, Jo, etc.