The Insular Celtic hypothesis groups the Celtic languages of the British Isles into:

• Goidelic languages (which include Irish, Manx, and Scottish Gaelic); and

• Brythonic languages (of which the modern descendants are Breton, Cornish and Welsh).

The nomenclature "Insular" refers to the location of the areas where these languages have been traditionally spoken, that being the British Isles. It therefore also refers to the notion that the Brythonic and Goidelic languages evolved together in those islands, having a common ancestor more recent than any shared with the Continental Celtic languages (Celtiberian, Gaulish and Lepontic among others, all of which are long extinct).

The proponents of the Insular Celtic hypothesis point to shared innovations among Insular Celtic languages, including inflected prepositions and VSO word order. They assert that a partition that lumps the Brythonic languages and Gaulish (P-Celtic) on one side and the Goidelic languages with Celtiberian (Q-Celtic) on the other may be a superficial one, as the identical sound shift (Q to P) could have occurred independently in the predecessors of Gaulish and Brythonic.

The family tree of the Insular Celtic languages is thus as follows:

• Insular Celtic
  • Goidelic
    • Primitive Irish, ancestral to:
    • Old Irish, ancestral to:
    • Middle Irish, ancestral to:
      • Irish
      • Scottish Gaelic
      • Manx
      • Galwegian
  • Brythonic
    • Cumbric
    • Pictish (possibly)
    • Old Welsh, ancestral to
    • Middle Welsh, ancestral to:
      • Welsh
    • Southwestern Brythonic, ancestral to:
      • Breton
      • Cornish

Celtic languages

Inselkeltische Sprachen | Celtique insulaire | 海島凱爾特語支