Power transmission belts are used to transfer mechanical power. They include V-belts, timing belts, and flat belts. V-belts have tapered cross-sections that lodge into the grooves of V-belt pulleys to transmit high torque and reduce slipping caused by under-tensioning. Timing belts have external teeth that interlock with the teeth of timing belt pulleys. They are either single-sided or double-sided. Flat belts are used with pulleys that have crowned surfaces to prevent them from slipping under heavy loads.