Abstract: A positioning device for positioning a part, such as, a movable part of a machine tool, relative to a fixed part of the machine tool, comprises, a transmitter adapted to be secured to one of said fixed and movable parts and having spaced apart first and second transmitter surfaces and a differential transmitter adapted to be secured to the other of the first and second parts and having first and second heads adapted to be positioned in alignment with the respective first and second transmitter surfaces, so that the heads produce a control signal in accordance with the amount of the heads aligned with the surfaces. Control means are connected to the respective heads and to means for moving a machine part to position the machine part in accordance with the coverages of the heads on the first and second surfaces.