Pure-blood is the term for wizards and witches who have a purely or approximately pure magical heritage. This means that there are no Muggles in their family trees, but this is rarely (if ever) true. If families are traced back all the way, for if wizards didn't mate with Muggles, their race would die out. As the only way for a family to retain their pure-blood status is to marry other pure-bloods, all pure-blood families were related to one another by the 1990s. Because of inbreeding among pure-bloods and some marrying Muggles and Muggle-borns, the number of families claiming to be pure-blood is declining. They are the least common blood type in the magical world.