The division of Celtic languages into Q- and P-families depends on whether they retained the Indo-European qu-or substituted a p-. The substitution of p- for qu- probably took place in the first millennium bc in central Europe and spread to the west, but not as far as Ireland or the Celtic areas of the Iberian peninsula. Gaulish was largely a P-Celtic language, with traces of Q-Celtic. The p-/ q- split is clearest in cognates retaining the same roots, e.g. W pen, head and Ir. ceann, head. The Modern P-Celtic languages are Welsh, Cornish, and Breton; these are also called Brythonic. The languages of ancient Britain and Scotland at the time of the Roman invasion were dominantly P-Celtic. See also BELGAE; BUILG; CALEDONII; ÉRAINN; PICTS.