Roller skating, popular source of sport and recreation. Traditional skates consist of four wheels fitted with ball bearings attached, 2 in front and 2 in back, to a shoe or to a steel platform that can be attached to a shoe. Clamp-on skates with metal wheels are generally worn out-of-doors while bootskates are most often used on rinks. For racing, wooden wheels are favored; for figure skating and roller hockey, wheels are usually made of plastic. In the late 1980s, a new type of skate, the roller blade, was introduced, with four wheels placed front to back to form a single line, or blade, of wheels.