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Southwestern Brythonic is one of two dialects into which the Brythonic language split following the Battle of Deorham in A.D. 577, the other being Western Brythonic, which later evolved into Welsh and Cumbric.

It is the common ancestor of Cornish and Breton, which in the opinion of some (such as Schrijver) did not become distinct before the 12th century, the terms "Old Cornish" and "Old Breton" being geographical rather than linguistic.

Some of the sound changes that distinguish Southwestern Brythonic from Welsh include:

• the raising of */(g)wo-/ to /(g)wu-/ in a pretonic syllable (in Welsh there was no raising)
• the fronting of */ɔː/ to /œː/ (in Welsh it diphthongized to /aw/)
• the fronting of */a/ to */e/ before */iː/ or */j/ in an old final syllable (in Welsh it diphthongized to /ei/)

Other significant differences are found in Welsh innovations that Southwestern Brythonic did not participate in, such as the development of the voiceless alveolar lateral fricative /ɬ/.

There has been some recent interest in the language, particularly in Devon, and this has included study of a booklet entitled A Handbook of West Country Brythonic: The Forgotten Celtic Tongue of South West England C.700 A.D. (Old Devonian) self published by Joseph Biddulph. Biddulph's work has been subject to criticism for not being sufficiently academic and for being effectively a constructed language rather than a reconstruction based on the comparative method.

• Celtic language family tree

• Jackson, Kenneth (1953). Language and History in Early Britain. Edinburgh: Edinburgh University Press. 
• Schrijver, Peter (1995). Studies in British Celtic Historical Phonology. Amsterdam: Rodopi. ISBN 90-5183-820-4.