Kingpins secure the casters' mounting plates to their frames on standard plate casters. They are the most popular form of caster used for general purpose applications and are employed in practically every sector. The mounting plates for the casters go on the bottom of machinery, carts, and other equipment, and the casters allow the equipment to roll so that moving or repositioning it is easier. There are swivel and rigid versions of these casters. Swivel casters can rotate 360 degrees, allowing equipment to shift directions while in motion. The casters are stiff and roll in a straight line. Two rigid plate casters are frequently combined with two swivel casters to allow the equipment to negotiate around curves and obstacles while moving while maintaining a straight path.