Western Brythonic was one of two dialects into which the British language split during the Early Middle Ages; its counterpart was Southwestern Brythonic. The reason and date for the split is often given as the Battle of Deorham in 577, at which point the victorious Saxons of Wessex essentially cut the Brythonic-speaking Britain in two.
Western Brythonic was spoken in Wales and the Hen Ogledd, or "Old North", the Brythonic-speaking area of northern England and southern Scotland. It became the ancestor to Old Welsh and the extinct dialect or language known as Cumbric, and thus to the modern Welsh language. Southwestern Brythonic became the ancestor to Cornish and Breton.