Appanage is important to the history of France during the Middle Ages. The law said that when a king dies, his oldest son becomes the next king. Kings who wanted to give a younger son some land too sometimes created an appanage. Appanage was one way to keep a civil war from happening, if two or more sons wanted to fight each other to be the next king.
The younger son controlled the appanage as a duke and ruled almost like a king, except he could not sell the land. The ruler of an appanage could give it to his sons and grandsons, but eventually that part of the family would run out of sons and the land would go back to the king.