Valves, in general, can be categorized into three types; directional control valves, flow control valves, and pressure control valves. This lesson discusses directional control valves. Directional control valves, in turn, can usually be classified into three different styles; sliding spool (1), rotary spool (2), and check valve (3). Sliding spool and rotary spools can be found in the hydraulic system as indicated above, while check valves can be incorporated into the directional control valve or as a separate component of the hydraulic circuit.