Power transmission belts are used to transfer mechanical power. They include V belts, round belts, timing belts, drive 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. Round belts are used in pulleys with a 60-degree V-groove. 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. They have largely been replaced by V-belts and timing belts.