Brittonic languages
Scholarly handling of the Celtic languages has been contentious owing to scarceness of primary source data. Some scholars (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) distinguish Continental Celtic and Insular Celtic, arguing that the differences between the Goidelic and Brittonic languages arose after these split off from the Continental Celtic languages. Other scholars (such as Schmidt 1988) distinguish between P-Celtic and Q-Celtic, putting most of the Gaulish and Brittonic languages in the former group and the Goidelic and Celtiberian languages in the latter. The P-Celtic languages (also called Gallo-Brittonic) are sometimes seen (for example by Koch 1992) as a central innovating area as opposed to the more conservative peripheral Q-Celtic languages.