The OHC system can be driven using the same methods as an OHV system, which include using a rubber/kevlar toothed timing belt, chain, or in less common cases, gears. In conjunction with multiple (3, 4 or 5) valves per cylinder, many OHC engines today employ variable valve timing to improve efficiency and power. OHC also inherently allows for greater engine speeds over comparable cam-in-block designs, as a result of having lower valvetrain mass. There are two overhead camshaft layouts: single overhead camshaft ("SOHC"), and double (or dual) overhead camshaft ("DOHC").