Szemerényi's law

Szemerényi's law is a Proto-Indo-European

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...

 phonological

Proto-Indo-European phonology

The phonology of the Proto-Indo-European language has been reconstructed by linguists, based on the similarities and differences among current and extinct Indo-European languages...

 rule, named after Hungarian linguist Oswald Szemerényi

Oswald Szemerényi

Oswald John Louis Szemerényi was a Hungarian Indo-Europeanist with strong interests in comparative linguistics in general....

, according to which word-final clusters of vowels (V), resonants (R) and of either */s/ or */h₂/ are simplified by dropping the word-final fricative (} was phonetically itself probably a back fricative), with 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...

 of the preceding vowel.

This law affected numerous nominative singulars of masculine and feminine nouns, as well as the nominoaccusatives of neuter collectives in }

Compare:

• PIE
  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...
  
    'father' > > Ancient Greek patḗr, Sanskrit pitā́

After the application of the law, the resulting word-final *-n would then be dropped, at least if the preceding segment was (unaccented) *ō. The resulting long vowels had already begun in PIE to spread analogically to other nominative singular forms in which they were not phonologically justified by the law (e.g. PIE 'foot'), and word-final sonorants other than *-n were sometimes dropped, which demonstrates that this law was already morphologized in the period of "PIE proper".