ブリソン諸語
Western Brittonic languages () 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.