The boukólos rule is a phonological rule of the Proto-Indo-European language (PIE). It states that a labiovelar stop (*kʷ, *gʷ, *gʷʰ) dissimilates to an ordinary velar stop (*k, *g, *gʰ) next to the vowel *u or its corresponding glide *w.
The rule is named after an example, the Ancient Greek word βουκόλος (bou-kólos; from Mycenaean Greekqo-u-ko-ro /gʷou̯kolos/[1]) "cowherd", ultimately from PIE *gʷou-kolos, dissimilated from *gʷou-kʷolos. If the labiovelar had not undergone dissimilation, the word should have turned out as *bou-pólos, as in the analogously constructed αἰπόλος (ai-pólos) "goatherd" < *ai(ǵ)-kʷolos.[2] The same dissimilated form *gʷou-kolos is the ancestor of Proto-Celtic*bou-koli-, the source of Welshbugail (which would have had -b- rather than -g- if it had come from a form with *-kʷ-) and Irishbuachaill, which is the common word for "boy" in the modern language.[3]
Another example could be the Greek negation οὐκ[ί] (ouk[í]), which Warren Cowgill has interpreted as coming from pre-Greek *ojukid < *(ne) h₂oju kʷid, meaning approximately "not on your life". Without the boukólos rule, the result would have been *οὐτ[ί] (out[í]).[4]
The rule is also found in Germanic, mainly in verbs, where labiovelars are delabialised by the epenthetic -u- inserted before syllabic resonants:[5]
Old High German queman ("to come"), past participle cuman ("come"), from Proto-Germanic *kwemaną and *kumanaz
Gothic saiƕan, Old High German sehan ("to see"), past plural OHG sāgun ("saw"), from Proto-Germanic *sehwaną and *sēgun (-g- results from earlier -k- through Verner's law)
See also
Weise's law, a similar Proto-Indo-European sound law affecting the palatovelar consonants
^Ringe, Donald A. (2017). From Proto-Indo-European to Proto-Germanic. Linguistic history of English, v. 1 (2nd ed.). Oxford: Oxford University Press. pp. 111–112. ISBN0-19-955229-0.