Oath of Strasbourg

In 842, [Louis the German]?, ruler of the eastern [Frankish kingdom]?, met with his brother, [Charles the Bald]?, ruler of the western Frankish kingdom at Strasbourg. At this meeting, Louis and Charles pledged their allegiance to each other -- and their opposition to the Emperor, their elder brother Lothar?. According to our chief source for the meeting, Nithard?'s Life of Louis the Pious, each king swore the oath in the vernacular of the other's kingdom. Historians have long used this passage to illustrate the theory that, by 842, Carolingian society had begun to split into separate proto-countries with different languages and customs.

In recent years, however, another theory has come to the front: the Carolingian Empire was comprised of several regna (loosely translated as "kingdoms") that had always maintained different customs and dialects. In support of this theory is the fact that both Charlemagne and Louis the Pious sent their sons to be raised in the regna they were meant to inherit as their primary territory, in order to guarantee the support of the people by being familiar with them and their customs.