In the French royal family, the courtesy title Monsieur was reserved for the king's brother; if the king had more than one brother, then the one closest to him in age used this title. The wife of Monsieur was called Madame, his eldest daughter Mademoiselle.
The two most famous holders of this title were Gaston de France, duc d'Orléans, son of Henri IV, and his nephew, Philippe de France, duc d'Orléans, son of Louis XIII.