The Southwestern Brythonic languages are the Brythonic

Brythonic languages

The Brythonic or Brittonic languages form one of the two branches of the Insular Celtic language family, the other being Goidelic. The name Brythonic was derived by Welsh Celticist John Rhys from the Welsh word Brython, meaning an indigenous Briton as opposed to an Anglo-Saxon or Gael...

 Celtic

Celtic languages

The Celtic languages are descended from Proto-Celtic, or "Common Celtic"; a branch of the greater Indo-European language family...

 tongues spoken in Southwestern Britain and Brittany

Brittany

Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

 since the Early Middle Ages

Early Middle Ages

The Early Middle Ages was the period of European history lasting from the 5th century to approximately 1000. The Early Middle Ages followed the decline of the Western Roman Empire and preceded the High Middle Ages...

. During the period of their earliest attestation, the languages appear to be indistinguishable, but eventually they evolved into the Cornish

Cornish language

Cornish is a Brythonic Celtic language and a recognised minority language of the United Kingdom. Along with Welsh and Breton, it is directly descended from the ancient British language spoken throughout much of Britain before the English language came to dominate...

 and Breton

Breton language

Breton is a Celtic language spoken in Brittany , France. Breton is a Brythonic language, descended from the Celtic British language brought from Great Britain to Armorica by migrating Britons during the Early Middle Ages. Like the other Brythonic languages, Welsh and Cornish, it is classified as...

 languages. They evolved from the common British language formerly spoken all across Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

, and were thus related to the Welsh

Welsh language

Welsh is a member of the Brythonic branch of the Celtic languages spoken natively in Wales, by some along the Welsh border in England, and in Y Wladfa...

 and Cumbric dialects spoken in Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 and the Hen Ogledd

Hen Ogledd

Yr Hen Ogledd is a Welsh term used by scholars to refer to those parts of what is now northern England and southern Scotland in the years between 500 and the Viking invasions of c. 800, with particular interest in the Brythonic-speaking peoples who lived there.The term is derived from heroic...

 (northern Britain), respectively.

The earliest stage of the languages, Primitive Cornish/Primitive Breton, is unattested.