A differential is a mechanical connection that translates rotational motion from a source to two axles. The driveshaft generally runs the length of the vehicle, from the energy input (a motor) to the energy output (the wheels). The axle of the driving wheels is split, with the driveshaft transferring motion through gearing arrangements to maintain a uniform direction and speed of the (live) axles. This arrangement is very common in rear-wheel drive cars and trucks, but is also employed in aircraft, production machinery and equipment, trains, and ships.