Szemerényi's law (Hungarian pronunciation: [ˈsɛmɛreːɲi]) is both a sound change and a synchronic phonological rule that operated during an early stage of the Proto-Indo-European language (PIE).
The rule deleted coda fricatives *s or laryngeals *h₁, *h₂ or *h₃ (cover symbol *H), with compensatory lengthening occurring in a word-final position after resonants.
In other words: The law affected the nominative singular forms of the many masculine and feminine nouns whose stem ended in a resonant: The rule also affected the nominative-accusative forms of neuter plural/collective nouns, which ended in *-h₂: Also in the third-person plural perfect ending: Compare: According to another synchronic PIE phonological rule, word-final *n was deleted after *ō, usually by the operation of Szemerényi's law: The PIE reconstruction for "heart" is the single instance where *d is deleted after *r, with compensatory lengthening of the preceding vowel.
The word-final sonorants other than *-n were sometimes dropped as well, which demonstrates that this law was already morphologized in the period of "PIE proper", and the long vowel produced was no longer synchronically viewed as the outcome of a process of fricative deletion.
Exceptions to Szemerényi's law are found in word-final: as well as medial positions: The forms without a laryngeal are considered to be more archaic and were likely to have been lexicalized at a later stage of PIE.