Cowgill's law (Greek)

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Cowgill's law says that a former /o/ vowel becomes /u/ between a resonant (/r/, /l/, /m/, /n/) and a labial consonant (including labiovelars), in either order. It is named after Indo-Europeanist Warren Cowgill.

Examples:

Greek: νύξ "night" < PIE *nokʷts (cf. Latin: nox, Ved. nák < *nakts, Gothic: nahts, gen. sg. Hittite: nekuz /nekʷts/)
Greek: φύλλον "leaf" < PIE *bʰolyom (cf. Latin: folium)
Greek: μύλη "mill" < PIE *mol-eh₂- (cf. Latin: molīna)
Greek: ὄνυξ "nail" (stem Greek: ónukh-) < early PG *onokʷʰ- < PIE h₃nogʷʰ- (cf. Old English: nægl < PGerm *nag-laz)

Note that when a labiovelar adjoins an /o/ affected by Cowgill's law, the new /u/ will cause the labiovelar to lose its labial component (as in Greek: núks and Greek: ónuks/ónukh-, where the usual Greek change */kʷ/ > /p/ has not occurred).

References

Sihler, Andrew L. (1995). New Comparative Grammar of Greek and Latin. Oxford University Press. ISBN 0-19-508345-8.