A three-way air valve can serve the on and off function of a two-way valve. However, in some applications, such as a pneumatically-controlled cylinder or actuator, there is the need to release air in order to allow the device to partially or fully retract.
Most three-way valves are normally either open or closed. When actuated, a normally open valve blocks the inlet port and opens the exhaust port. When not actuated, a normally open valve blocks the exhaust port and opens the outlet port.
When actuated, a normally closed valve blocks the exhaust port and opens the inlet port. When not actuated, a normally closed valve blocks the inlet port and opens the exhaust port.