Spool valves (2): Each of the spool valve symbols shown have different center oil paths. The symbol on the left indicates a closed-center valve, where all the ports are normally blocked. The center symbol represents a tandem-center valve, where the flow from the pump will return to the tank, and both the work ports are blocked. The tandem center valve is normally referred to as an open-center valve in Caterpillar Training Literature. The symbol on the right shows an open-center path, where oil freely flows to all available connections.