In the form used on aircraft carriers the catapult consists of a track, or slot, built into the flight deck, below which is a large piston or "shuttle" that is attached through the track to the nose gear of the aircraft, or in some cases a wire rope, called a catapult bridle, is attached to the aircraft and the catapult shuttle. Other forms have been used historically, such as mounting a launching cart holding a seaplane on a long girder-built structure mounted on the deck of a warship or merchant vessel, but most catapults share a similar sliding track concept.