Purebred dog refers to a dog of a modern dog breed that closely resembles other dogs of the same breed with ancestry documented in a stud book and registered with one of the major dog registries. Documentation (so that the dog is known to be descended from specific ancestors) and registration distinguish modern breeds from dog types or landraces of dog (sometimes called natural breeds or ancient breeds) that arose under human influence over a long period of time to do a specific type of work.