Air valves allow metered flow of fluid in one or both directions. Many of them allow for free flow in one direction and reduced or metered flow in the reverse direction. They are used in pneumatic circuits to regulate the rate of activation or exhaust of cylinders and other pneumatic devices.  Air valves can be configured in-line or at right angles.  In an in-line configuration the input and output ports are on the same axis.  In a right angle configuration the input and output ports are perpendicular.