Casters are complete wheel and horn assemblies that attach to equipment to allow movement. There are two basic types of industrial casters: rigid and swivel. Rigid casters permit only straight-line movement. Swivel casters allow movement in any direction. Some varieties of swivel casters also include a lock that, when engaged, transforms the swivel into a rigid-style caster. Both rigid and swivel industrial casters are made out of materials such as aluminum, brass, chrome, copper, iron, steel, plastic, and rubber. In some cases, the wheel or roller is made of one type of material while the plate, horn, and mounting flange are made of another. Typically, industrial casters are used on trucks and trailers, dollies, and drum-handling equipment. Important specifications to consider include load capacity, wheel tread width, wheel diameter, and height.