DOHC stands for 'Double Overhead Camshafts' whereas SOHC refers to 'Single Overhead Camshafts'. In DOHC configurations, there are two camshafts on top of the cylinder head, one operating the inlet valves and one operating the exhaust valves. Some engines have more than one bank if cylinder heads ( like a V8) and these have more than two camshafts in total, but they are sill referred to as DOHC. The advantages of such a setup is that the placement of this park plug can be in the middle of the combustion chamber, thus improving the efficiency of the engine. The absence of a rocker in DOHC engine also reduces the mechanical power loss.