The County of Rouen was a medieval Viking fief and the forerunner of the Duchy of Normandy. In 911, Charles III, (King of France and known as 'Charles the Simple') made peace with the Viking leader Rollo in the Treaty of Saint-Clair-sur-Epte. The treaty granted Rollo the County of Rouen. Between the creation of the County and the later creation of the Duchy of Normandy, many Vikings settled in the region, leaving an enduring legacy in the form of the Cauchois dialect and the ethnic makeup of the Cauchois Normans.