“The basic change was the famous substitution of “Q”, the sound now represented by a hard “C”, into “P”. To give a simple example, the word for “son” in Irish is mac, in Welsh this became map and in modern Welsh is shortened to ap. “Everyone”, or cách, in Old Irish, is paup in Old Welsh. The word for a “feather” in Old Irish, clúmh, became pluf in Old Welsh. Thus the “Q” is substituted for the “P” and hence the identification of “P” and “Q” Celtic and perhaps the origin of the phrase about “minding your ‘p’s’ and ‘q’s’ ”.”