Strictly speaking, the American pit bull terrier isn't a proper breed, and strictly speaking the pit bull terrier is also illegal under the Dangerous Dogs Act. Hence the development of the semi-illegal pit bull cross.
For those who like their dogs intimidating, unpredictable and vicious, there are many options, and the pit bull stock has been being carefully cross-bred with other powerful doggy types with a Mendelian attention to desirable characteristics that the 18th-century pioneers of selective breeding would recognise.
Various mastiffs – the Staffordshire bull terrier and, of course, the Rottweiler, the original devil dog – are used to modify the pit bull, but only those specimens with the nastiest temperaments are chosen to create something that may end up chewing the owner's three-year-old nephew to death as a helpless blood-spattered granny or aunty looks on. Such fatal attacks remain rare, but are now an established risk of life in the United Kingdom's inner-city post codes.