The "Insular Celtic hypothesis" is a theory that the Brittonic and Goidelic languages evolved together in those islands, having a common ancestor more recent than any shared with the Continental Celtic languages such as Celtiberian, Gaulish, Galatian and Lepontic, among others, all of which are long extinct.
The proponents of the Insular Celtic hypothesis (such as Cowgill 1975; McCone 1991, 1992; and Schrijver 1995) point to shared innovations among Insular Celtic languages, including inflected prepositions, shared use of certain verbal particles, VSO word order, and the differentiation of absolute and conjunct verb endings as found extensively in Old Irish and to a small extent in Middle Welsh (see Morphology of the Proto-Celtic language). They assert that a partition that lumps the Brittonic languages and Gaulish (P-Celtic) on one side and the Goidelic languages with Celtiberian (Q-Celtic) on the other may be a superficial one (i.e. owing to a language contact phenomenon), as the identical sound shift (/kʷ/ to /p/) could have occurred independently in the predecessors of Gaulish and Brittonic, or have spread through language contact between those two groups.
The family tree of the Insular Celtic languages is thus as follows:
The following table lists cognates showing the development of Proto-Celtic */kʷ/ to /p/ in Gaulish and the Brittonic languages but to /k/ in the Goidelic languages.
A significant difference between Goidelic and Brittonic languages is the transformation of *an, am to a denasalised vowel with lengthening, é, before an originally voiceless stop or fricative, cf. Old Irish éc "death", écath "fish hook", dét "tooth", cét "hundred" vs. Welsh angau, angad, dant, and cant. Otherwise: