Grand Prix motorcycles are purpose-built racing machines that are unavailable for purchase by the general public and unable to be ridden legally on public roads. This contrasts with the various production-based categories of racing, such as the Superbike World Championship and the Isle of Man TT Races that feature modified versions of road-going motorcycles available to the public. The current top division is known as MotoGP since when the four-stroke era began. Prior to that, the largest class was 500cc, both of which form a historical continuum as the official World Championship, although all classes have official status.