The division of the 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. The p-/q- split is clearest in cognates retaining the same roots, e.g. Ir. ceann, head and W pen, head. The modern Q-Celtic languages are Irish, Scottish Gaelic, and Manx; these are also called Goidelic. The distinctions are not always absolute; Irish Q-Celtic speakers settled in Dyfed, a Welsh-speaking or P-Celtic region. See GOÍDEL GLAS.