Strasbourg, Oath of, 842, oath sworn by Charles the Bald (later Holy Roman Emperor Charles II) and Louis the German in solemnizing their alliance against their brother, Emperor Lothair I. The chief political result of this alliance was the Treaty of Verdun (843; see Verdun, Treaty of). Each brother made his oath in the language of the other's followers, so that it might be understood. The version used by Louis is often considered the oldest known specimen of French.