Most wheelchairs use four wheels, with two large wheels at the back and two smaller ones (casters) at the front. The standard tire used for the rear wheels on most wheelchairs is a pneumatic tire, for which the standard size is 24 inches. Smaller and larger sizes, however, also are available. Many manufacturers now also offer other types of tires–such as solid tires, semi-pneumatic, or radial tires–at extra cost. Mag wheels and off-road wheels also are options on some chairs. Casters, too, vary in size (ranging from six to eight inches in diameter) and composition (pneumatic, solid rubber, plastic, or a combination of these).