Southwestern Brittonic languages
The Southwestern Brittonic languages (Cornish: Brythonek Dyghowbarthgorlewin, Breton: Predeneg Kreisteizkornôg) are the Brittonic Celtic languages spoken in what is now South West England (later confined to Cornwall), and Armorica (later confined to Brittany) now in France, since the Early Middle Ages. During the period of their earliest attestation, the languages appear to be indistinguishable, but they gradually evolved into the languages Cornish (influenced by Welsh and English), and Breton (influenced by oïl French). Both languages evolved from the Common Brittonic formerly spoken across most of Britain and were thus related to the Welsh and Cumbric varieties spoken in Wales and Hen Ogledd (the Old North, i.e. Northern England and the Scottish Lowlands), respectively. The older term Southwestern Brythonic languages is sometimes also used.
The earliest stage of the languages, Primitive Cornish/Breton, is unattested. Written sources are extant from the Old Cornish/Breton period, roughly 800–1100, in which phase the languages are indistinguishable. As such, some linguists such as Peter Schrijver use the term Southwest British (i.e. Southwest Brittonic) to describe the language when "Old Cornish" and "Old Breton" were indistinguishable and only separated by geography rather than linguistically.