There is no universal system for classifying all types of motorcycles. There are strict classification systems enforced by competitive motorcycle sport sanctioning bodies, or legal definitions of a motorcycle established by certain legal jurisdictions for motorcycle registration, emissions, road traffic safety rules or motorcyclist licensing. There are also informal classifications or nicknames used by manufacturers, riders, and the motorcycling media. Some experts do not recognize sub-types, like naked bike, that "purport to be classified" outside the six usual classes, because they fit within one of the main types and are recognizable only by cosmetic changes.