Common Celtic retained the Indo-European k^w and g^w sounds. In the language groups where these sounds are preserved, k^w tends to p and g^w tends to b. Celtic had a b which easily attracted the g^w but it had lost the Indo-European p sound. thus the transition from k^w to p involved the introduction of a new phonetic element.

This happened in the continental Celtic languages with the exception of Celt-Iberian, and in the insular Celtic branch with the exception of Goidelic. Both resisted the change (possibly due to their relative isolation) and are consequently know as "q" Celtic languages.