The Cooper Mk.X, and its evolutions, the Mk.XI, the Mk.XII, and the Mk.XIII, are open-wheel Formula Three race cars, designed, developed, and built by British manufacturer Cooper in 1956 (with production continuing through 1959).^[1] It was virtually identical in design to the previous Mk.IX. The internal designation, dubbed the T42,^[2] was powered by a 500 cc (31 cu in) JA Prestwich Industries (JAP) single-cylinder engine, and featured a singular brake disk at the rear of the car, flatter springs to mitigate ground clearance, altered and adjusted center spring mountings, and reworked engine mounts.^[3] The second version featured an elongated chassis and body, and a larger and more powerful 1,000 cc (61 cu in) OHV V-2 engine. Weight and chassis dimensions were essentially identical to the previous model.^[4]