 Sometimes, a single idler gear is used to reverse the direction, in which case it may be referred to as a reverse idler.For instance, the typical automobile manual transmission engages reverse gear by means of inserting a reverse idler between two gears.Since a driven gear (gear "A") rotating clockwise will drive a second gear ("B") counterclockwise, adding a third gear to the string means that gear "C" will be spinning the same direction as "A".