A simple or single-geared lathe has a straight train of gearing from its spindle to its feed screw, with intermediate gears which serve as idlers to take up the distance between the driver and driver gears or spindle and screw gears. Index plates, giving the change of gear used for different threads, are usually found on lathes, but when threads are called for that are not indexed or when those ending in fractions are to be cut, the machinist must resort to his own figures.

The term compound in lathe practice means that in the train of gearing from the spindle to the lead screws of the lathe, there is a stud or spindle with two different sized gears. These gears are so connected as to change the link of revolution between the spindle and the lead screw to a different number of revolutions from that which would take place if the straight line of gears were used.