Comment by thereisnospork

>Timing belts have teeth.

To expand: It is the teeth that make belts timing belts, they keep the 'timing' (relative rotational orientation) of 2 or more toothed pulleys. In an ICE the camshafts are locked to a 2:1 ratio to the crank shaft, in a 3d printer it keeps an axis fixed relative to the stepper motor shaft.