Variable diameter pulleys must always appear in pairs. One of the pulleys, called the drive pulley, is connected to the crankshaft of the engine. The drive pulley is also called the input pulley, because it is the pulley that transmits the energy of the engine to the transmission. The other pulley is called the driven pulley, because it is rotated by the first pulley. The driven pulley acts as an output pulley, which can transmit energy to the drive shaft.
The distance between the center of the pulley and the contact position of the belt in the groove is the pitch radius. When the pulley is far away, the belt is located lower and the pitch radius decreases. When the pulley approaches, the belt is located higher and the pitch circle radius increases. The ratio of the pitch circle radius of the driving pulley to the pitch circle radius of the driven pulley determines the level of the gear.