The Celts represented Indo-European q by p, whilst the Greeks, Illyrians, Thracians, Ligurians, and aborigines of France, Britain and Ireland represented it by k, c or qu.
Many languages find the combination qu, when both sounds are consonantal (qw), difficult; q being the deepest guttural while u (English w) is a lip sound, the points of production are nearly as far separate as they can be.
In the Goidelic group qu appears as c, thus Irish cethir, " four "; in the Brythonic group it is changed into p, as in Welsh pedwar, " four."
The velar tenuis q, when labialized, became qu, without labialization became k; the velar media g became b or g.
In Brythonic, primitive Celtic qu became p, as above noted.