Cowgill's law

Etymology
Named after Indo-Europeanist.

Proper noun

 * 1)  A law of sound change, stating that a former  vowel in Proto-Greek becomes  between a resonant  and a labial consonant (including labiovelars), in either order.
 * 2)  A law of sound change, stating that a Proto-Indo-European laryngeal, and possibly , turns into  when directly preceded by a sonorant and followed by.