The boukólos rule is a phonological rule of the Proto-Indo-European language (PIE). It states that a labiovelar stop dissimilates to an ordinary velar stop next to the vowel or its corresponding glide. The rule is named after an example, the Ancient Greek word βουκόλος (bou-kólos; from Mycenaean Greek qo-u-ko-ro /gʷou̯kolos/ ) "cowherd", ultimately from PIE, dissimilated from. 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" <. The same dissimilated form is the ancestor of Proto-Celtic, the source of Welsh bugail (which would have had -b- rather than -g- if it had come from a form with *-kʷ-) and Irish buachaill, which is the common word for "boy" in the modern language. Another example could be the Greek negation οὐκ[ί] (ouk[í]), which Warren Cowgill has interpreted as coming from pre-Greek *ojukid <, meaning approximately "not on your life". Without the boukólos rule, the result would have been *οὐτ[ί] (out[í]). The rule is also found in Germanic, mainly in verbs, where labiovelars are delabialised by the epenthetic -u- inserted before syllabic resonants: