P-Celtic and Q-Celtic languages
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The P-Celtic/Q-Celtic hypothesis is a categorization for the Celtic languages. The theory links Gaulish with Brythonic as P-Celtic and links Goidelic with Celtiberian as Q-Celtic. The difference between P and Q languages is the treatment of Proto-Celtic *kw, which became *p in the P-Celtic languages but *k in Goidelic. For example the word for head is pen in Brythonic languages but ceann in Goidelic; the word for son is mab (earlier map) in Brythonic but mac in Goidelic – maqq on the Primitive Irish Ogham inscriptions.
P-Celtic incorporates the following:
- Brythonic (British)
Q-Celtic incorporates the following:
- Celtiberian languages
Alternative theories 
A competing scheme was defended for example by McCone (1996), links Goidelic and Brythonic together as an Insular Celtic branch, as opposed to Continental Celtic languages including Gaulish and Celtiberian.