Stang's law is a Proto-Indo-European (PIE) phonological rule named after the Norwegian linguist Christian Stang.
The law governs the word-final sequences of a vowel, followed by a semivowel ( or ) or a laryngeal (), followed by a nasal. According to the law these sequences are simplified such that laryngeals and semivowels are dropped, with compensatory lengthening of a preceding vowel.
This rule is usually cited in more restricted form as: and ( denoting a vowel and a long vowel).
Often the rules and also are added: