The rotation of the engine is also partially harnessed via a pulley system to run a number of other systems in the car. There are two main types of car pulleys: crank pulleys, and accessory pulleys. Most pulleys are driven by a main crank pulley, which is bolted onto the crankshaft. When the engine is operating, the crank pulley rotates, transferring motion to the other pulleys by means of the serpentine belt or v belt. Accessory pulleys are mounted on the accessories like the AC Compressor, Alternator, Water Pump etc. and rotational motion enables them to operate. There are also idler pulleys and tensioner pulleys and they do not operate accessories but present in the system to form the complete path of the pulley system. Tensioner pulleys works to maintain the proper tension in the system.