King William [the Conqueror] had been, and still continued to be, as bad a vassal to his liege lord, the king of France, as it was possible to imagine; but he had no intention of being so treated by any of his own vassals. . . . William, as duke of Normandy, held the allegiance of his vassals and let them "legally" against the French king; William as king of England saw to it that no vassal could do the same to him.
(from A History of England and the British Empire]