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 (; from Mycenaean Greek qo-u-ko-ro ) "cowherd", ultimately from Proto-Indo-European , dissimilated from . If the labiovelar had not undergone dissimilation, the word should have turned out as *, as in the analogously constructed () "goatherd" < . The same dissimilated form is the ancestor of Proto-Celtic , the source of Welsh (which would have had rather than if it had come from a form with ) and Irish , which is the common word for "boy" in the modern language.
Another example could be the Greek negation (), which Warren Cowgill has interpreted as coming from pre-Greek < , meaning approximately "not on your life". Without the boukólos rule, the result would have been * ().
The rule is also found in Germanic, mainly in verbs, where labiovelars are delabialised by the epenthetic -u- inserted before syllabic resonants: Old High German ("to come"), past participle ("come"), from Proto-Germanic and Gothic , Old High German ("to see"), past plural OHG ("saw"), from Proto-Germanic and ( results from earlier through Verner's law)
See also Glossary of sound laws in the Indo-European languages Weise's law, a similar Proto-Indo-European sound law affecting the palatovelar consonants
References )