Stang's law

Stang'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 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 laryngeals and semivowels are dropped, 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 a preceding vowel.

This rule is usually cited in more restricted form as: *Vwm > *V:m. Often the rules *Vmm > *V:m and } > *V:m; and also *Vyi > *V:y are added.

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...
  
  'sky' (accusative singular) > *dyḗm > Sanskrit dyā́m, acc. sg. of dyaús, Latin diem (which served as the basis for Latin dies 'day'), Greek Zen (reformed after Homeric Greek to Zena), acc. of Zeus
• PIE } 'cow' (acc. sg.) > } > Sanskrit gā́m, acc. sg. of gaús, Greek (Homeric and dialectal) bṓn, acc. sg. of bous 'cow'
• accusative singular of PIE } 'house' is , not .