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 Greek, Ancient (to 1453);: βουκόλος (; from Mycenaean Greek qo-u-ko-ro /gʷou̯kolos/[1]) "cowherd", ultimately from PIE, dissimilated from . If the labiovelar had not undergone dissimilation, the word should have turned out as *, as in the analogously constructed Greek, Ancient (to 1453);: αἰπόλος "goatherd" < . The same dissimilated form is the ancestor of Proto-Celtic, the source of Welsh Welsh: bugail (which would have had -b- rather than -g- if it had come from a form with *-kʷ-) and Irish Irish: buachaill, which is the common word for "boy" in the modern language.[2]
Another example could be the Greek negation Greek, Ancient (to 1453);: οὐκ[ί], which Warren Cowgill has interpreted as coming from pre-Greek Indo-European languages: *ojukid <, meaning approximately "not on your life". Without the boukólos rule, the result would have been *Greek, Ancient (to 1453);: οὐτ[ί] .
The rule is also found in Germanic, mainly in verbs, where labiovelars are delabialised by the epenthetic -u- inserted before syllabic resonants:[3]