Co-Ax valves incorporate two elements, one inside the other. The external element or valve housing including the valve seat, is stationary. In order to shift the valve, the internal element or control tube is actuated, either by electrical power (direct-acting) or air (externally-controlled). This co-linear design is what gives rise to the term coaxial valve. The external housing may have two or more inlets and outlets. The internal control element or control tube moves open or closed between the inlets and outlets and seats against an obturator style self-compensating seat. Co-ax valves are pressure-balanced. Internal piloting is not required for valve operation.