Variable-diameter pulleys must always come inpairs. One of the pulleys, known as the drive pulley（or driving pulley）, is connected to the crankshaftof the engine. The second pulley is called the drivenpulley. As an output pulley, the driven pulleytransfers energy to the driveshaft（See fig. 2.4.2）.
　　When one pulley increases its radius, the otherdecreases its radius to keep the belt tight. As the twopulleys change their radii relative to one another,they create an infinite number of gear ratios —— fromlow to high and everything in between. Forexample, when the radius is small on the drivingpulley and large on the driven pulley, then the rotational speed of the driven pulley decreases,resulting in a lower "gear. " When the radius is large on the driving pulley and small on the drivenpulley, then the rotational speed of the driven pulley increases, resulting in a higher "gear. "Thus, in theory, a CVT has an infinite number of "gears" that it can run through at any time, atany engine or vehicle speed.
　　The simplicity and stepless nature of CVTs make them an ideal transmission for a variety ofmachines and devices, not just cars. CVTs have been used for years in power tools and drillpresses. Theyve also been used in a variety of vehicles, including tractors, snowmobiles andmotor scooters. In all of these applications, the transmissions have relied on high-density rubberbelts, which can slip and stretch, thereby reducing their efficiency.