An RPM measures the rotation speed of a shaft or disk, as in a motor or tractor engine. RPM signifies the revolutions per minute (RPM) on a calibrated analog dial, but digital displays also exist and are common in modern vehicles. In the automotive world, one is used for engine revolutions and the other for vehicle speed. In tractors, the RPM is displayed with a green arc showing the speed range in which the engine produces maximum torque, which is the main focus of operators of tractors. Moreover, Tractors equipped with a power take-off (PTO) system have RPM, which shows the engine speed needed to rotate the PTO at the standardized speed required by most PTO-driven implements.