See, I’ve been doing research on the French artistocracy. I know a decent amount (for an American) about the British peerage system, but whenever I tried to transfer that knowledge to French novels I read, it never quiet worked. So, weird things I found out about French Aristocracy (pre-revolutionary; I haven’t tackled whatever Napoleon did):

• They have the Duc, Marquis, Comte, Vicomte, and Baron just like in England, but these titled people are rarely considered peers (or Pair as it’s said in French). This blew my brain. Pairie apparently comes from the idea of equality, so to say that you’re a Pair is to say that you are equal to the King. Only Princes who are also Ducs and some really old family Ducs can claim Duc et Pair, a process involving a letter from the King and approval of the governing body. Everybody else is “just” a noble.
• The Capetian Dynasty, which produced most of France’s kings, including Louis the XIV (the Bourbons are all Capetians), is Europe’s longest running continuous monarchy. Capet was elected King of France in 987, and his descendents have ruled, at various times, France, Spain, Luxemburg, Portugal, Poland, Hungary, Sciliy, Naples, and some other places I’ve never heard of. Currently, a Capet sits on the throne of Spain and another is the GrandDuke of Luxemberg (apparently that’s their King equivalent, as they don’t call anyone “king”). That’s over 1000 years of dynasty.  Anywho, since titled people don’t have last names, that’s why in the French Revolution, Louis XVI became known as Louis Capet, because then he had a name like everybody else.
• They like to double use titles. There are several people called “Princes” who are not related to the King. Prince du Sang, however, only designated people descended in the male line from a King. This used to apply to everyone who was directly descended through the male line from Hugh Capet, but the Bourbons excluded all (or most?) other Capetian families from the title.
• The church was involved in their peerage, so they had ecclesiastical and lay peers. The ecclesiastical peers supposedly were higher status than the lay peers.
• France frequently (always?) used agnatic seniority. I’d never heard of this before. It means that the King’s brother get the throne before his son (and women never inherit). This seems way more confusing to me. If I understand it correctly, if you’re the eldest son of the King’s brother, your father may become King, but as long as the current King has sons, you won’t (assuming no disaster). Being a student of British literature, I’m used to male-preference primogeniture, which means that the oldest son inherits, unless there’s no sons, in which case a woman could inherit. The brother thing only happens if there’s not children.
• On a non-French note, as of 1980, thanks to Sweden (go Swedes!) the world now has absolute cognative primogeniture, i.e. the eldest child, not eldest son, gets the throne. The Netherlands, Norway, Denmark, and Belgium have elected to do the same. And that’s just one more reason to love vikings (and Belgians).