ProtoBrythonic

ProtoBrythonic is the reconstructed common ancestor of the Brythonic branch of Insular Celtic, the lineage that gave rise to Welsh, Cornish, and Breton. As a linguistic construct, it is not directly attested in surviving texts but is inferred through the comparative method from features shared by its descendant languages and related Celtic languages. ProtoBrythonic is typically placed in the late pre-Roman to early Roman period, spoken in the western British Isles—principally in what is now Wales and Cornwall—and in Brittany, before these regions diverged into their distinct languages.

Historically, ProtoBrythonic sits on a major split within Celtic between Brythonic and Goidelic (Gaelic) branches. The Brythonic descendants spread through migration and cultural contact, with Breton reflecting continued ties to the British Isles after settlers moved to Brittany. The reconstruction relies on systematic correspondences among Welsh, Cornish, Breton, and related early forms, along with evidence from Latin and other contemporaneous sources. No direct writings in ProtoBrythonic survive; the language is inferred from later Brythonic languages and from broader Proto-Celtic data.

Linguistically, ProtoBrythonic is associated with innovations shared by its descendants, including the development of a mutation system that remains a hallmark of Brythonic languages, as well as consonant and vowel patterns that differentiate it from Goidelic. It also helps explain word formation, a shared morphology, and certain lexical layers retained or altered in Welsh, Cornish, and Breton.

ProtoBrythonic is central to studies of Celtic history, language contact in the British Isles, and the formation of the Brythonic linguistic identity.