Abstract: A timing belt having an outer section defined by cogged ribs defining a preselected controlled coefficient of friction obtained by providing controlled size grooves extending transversely through the ribs. The ratio of the effective side area of the grooves to the side area of the cogs defined in the ribs by the grooves, in the illustrated embodiment, is in the range of approximately 1/2 to 21/2 where the coefficient of the rib material is approximately 0.35. The cogs are raw edged and provided with a cover fabric. The provision of the cogged ribs permits the belt to be used as a double timing belt as well as providing improved functioning with backside rib pulleys. The effective coefficient of friction obtained by reducing the side area of the ribs as a result of the provision of the grooves is preferably in the range of 0.15 to approximately 0.90.