While the fleur-de-lis has appeared on countless European coats of arms and flags over the centuries, it is particularly associated with the French monarchy in a historical context, and continues to appear in the arms of the King of Spain and the Grand Duke of Luxembourg, members of the House of Bourbon. It remains an enduring symbol of France that appears on French postage stamps, although it has never been adopted officially by any of the French republics. According to French historian Georges Duby, the three leaves represent the medieval social classes: those who worked, those who fought and those who prayed.