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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 núks and ónuks/ónukh-, where the usual Greek change *kʷ > p has not occurred).
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 /h₃/, and possibly /h₂/, turns into /k/ in Proto-Germanic when directly preceded by a sonorant and followed by /w/. This law is still controversial, although increasingly accepted. Donald Ringe (2006) accepts it; Andrew Sihler (1995) is noncommittal.
Examples are fairly few:
If it becomes generally accepted, the relative chronology of this law could have consequences for a possible reconstructed phonetic value of h₃. Since Germanic /k/ results from earlier PIE /g/, and since the change occurred before Grimm's law applied (according to Ringe), the resulting change would be actually h₃w > gʷ. This would have been more likely if h₃ was a voiced velar obstruent to begin with. If h₃ was a voiced labiovelar fricative as is occasionally suggested, the change would therefore have been: ɣʷw > ɡʷ.