Breton is not thought to be a descendant of any of the Continental Celtic languages such as Gaulish (though it may have borrowed some features from it), rather, it is descended from the Brythonic branch of Insular Celtic languages brought by Romano-British settlers to Brittany after the Roman departure from Britain in the early 5th century. The modern-day language most closely related to Breton is Cornish, followed by Welsh. (The other regional language of Brittany, Gallo, is an Oïl language derived from Latin).
Breton is traditionally spoken in Lower Brittany, roughly to the west of a line linking Plouha and Vannes. It comes from a language community between Britain and Armorica (present-day Brittany). It was the language of the elite until the 12th century.