Timing belts are toothed. To provide examples, we use them to pull camshafts, or for the secondary transmission in some motorbikes. They are also used in many industrial and agricultural machines. This type of belt is key in helping to avoid phase differences between the inlet and the outlet.
Even when well tightened and with no sliding, a non-grooved belt will shift owing to its elasticity. In fact, the length of its stretch will differ between tight strands and slack strands, and this is the variation that causes the phase differences.