On a French license plate, the number on the bottom right indicates the département of the car. So all Paris cars have the number 75 there; Paris is département 75 (in alphabetical order, I believe). By the way, this is also why Paris postal codes have a 75. The postal code 75001, for example, tells you that the address is in Paris in the first arrondissement.
Since 2009, when the new plaques minéralogiques came in, car owners have had the choice to add a regional sticker above their département number. In this way, an Alsatian in Nice or a Basque living in Lille can still feel connected to home. At least that was the idea.
Although there are 26 regions, each with its own regional symbol, you don't ever see most of them in Paris. People don't have to choose a symbol. And for some reason, besides the one you'd expect-- Île-de-France's red star-- the only regional symbols you see a lot in Paris are the black-and-white stripes of Brittany, and the Moor's-head of Corsica.
I couldn't figure out why. One day when I expressed my puzzlement, someone said, "Oh! That's because people hope if they have the Moor's-head [which in French by the way also sounds like "skull"], the cops will leave them alone. Histoire de mafia!"
