A power take-off or power takeoff (PTO) is any of several methods for taking power from a power source, such as a running engine, and transmitting it to an application such as an attached implement or separate machines. Most commonly, it is a system comprising a splined output shaft on a tractor or truck, designed so that a PTO shaft, a kind of drive shaft, can be easily connected and disconnected, and a corresponding input shaft on the application end. The power take-off allows implements to draw energy from the engine. Semi-permanently mounted power take-offs can also be found on industrial and marine engines. These applications typically use a drive shaft and bolted joint to transmit power to a secondary implement or accessory. In the case of a marine application, such shafts may be used to power fire pumps. In aircraft applications, such an accessory drive may be used in conjunction with a constant speed drive. Jet aircraft have four types of PTO units, internal gearbox, external gearbox, radial driveshaft, and bleed air which are used to power engine accessories. In some cases, aircraft power take-off systems also provide for putting power into the engine during engine start. See also Coffman starter.