Cowgill's law
Cowgill's law, named after Indo-Europeanist Warren Cowgill, refers to two unrelated sound changes, one occurring in Proto-Greek and the other in Proto-Germanic. Cowgill's law in Greek
In Proto-Greek, Cowgill's law says that a former  vowel becomes  between a resonant (, , , ) and a labial consonant (including labiovelars), in either order. Examples:
  "night" < PIE  (cf. , Ved. < *nakts, , gen. sg. /nekʷts/)
  "leaf" < PIE  (cf. ) "mill" < PIE  (cf. ) "nail" (stem ) < early PG  < PIE  (cf. < PGerm )

Note that when a labiovelar adjoins an  affected by Cowgill's law, the new  will cause the labiovelar to lose its labial component (as in  and , where the usual Greek change * >  has not occurred). Cowgill's law in Germanic
Cowgill's law in Germanic has no relation to Cowgill's law in Greek other than having been named after the same person. It says that a PIE laryngeal , and possibly , turns into  in Proto-Germanic when directly preceded by a sonorant and followed by . This law is still controversial, although increasingly accepted. Donald Ringe (2006) accepts it; Andrew Sihler (1995) is noncommittal. Examples are fairly few:
 Proto-Germanic  "alive" (whence English quick) < PIE  (cf. ) Proto-Germanic  acc. du. "us two" (cf. , , ) < PIE  (cf. ; Ved. acc. du. "us two" < )
 Possibly  "husband's brother" < PIE  (cf. , Ved. , )

The first two examples, however, have good alternative explanations which don't involve Cowgill's law:
 Proto-Germanic  < PIE . Proto-Germanic  < PIE  acc./dat. du. "us two at least" (other accusative personal pronouns may have been built the same way: Proto-Germanic  acc. sg. "me",  acc. sg. "you (sg. )", and  acc./dat. du. "you two" ). If the sound law becomes generally accepted, the relative chronology of this law could have consequences for a possible reconstructed phonetic value of . Since Germanic  results from earlier PIE , and since the change occurred before Grimm's law applied (according to Ringe), the resulting change would be actually  > . This would have been more likely if  was a voiced velar obstruent to begin with. If  was a voiced labiovelar fricative as is occasionally suggested, the change would therefore have been:  > . Notes

References

Indo-European linguistics
Sound laws