Western Brittonic languages (Welsh: Brythoneg Gorllewinol) comprise two dialects into which Common Brittonic split during the Early Middle Ages; its counterpart was the ancestor of the Southwestern Brittonic languages. 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 Brittonic-speaking Britain in two, which in turn caused the Western and Southwestern branches to develop separately.^[1]

According to this categorisation, Western Brittonic languages were spoken in Wales and the Hen Ogledd, or "Old North", an area of northern England and southern Scotland. One Western language evolved into Old Welsh and thus to the modern Welsh language; the language of yr Hen Ogledd, Cumbric, became extinct after the expansion of the Middle Irish-speaking Dál Riata polity.^[2] Southwestern Brittonic became the ancestor to Cornish and Breton.^[2]

Alan James, however, has suggested a contrary model where Cumbric and Pictish were more closely aligned to one another than they were to Welsh.^[3]