In the 13c, the right of appeal available to any French *vassal to his lord's lord. It applied equally to the vassals of English kings in e.g. Gascony. Thus a French vassal could appeal over the English king's head. This was potentially a source of mischief to the English king, who was a vassal of the French king for his lands in France. The English kings sought to avoid the inherent complications, while the French king wished to perpetuate the situation as a means of controlling, in some small measure, the English.
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.