A duplex transmission path, circuit, or channel designed to support information transfer in both directions, simultaneously. An FDX circuit can be a single physical circuit, such as a voice grade local loop. Alternatively, an FDX circuit can comprise two simplex circuits, one operating in each direction. Traditionally, T 1 and E-1 circuits were provisioned over two simplex twisted pair circuits. A half-duplex (HDX) circuit differs in that it supports information transfer in both directions, but only one direction at a time. Most circuits are FDX in nature. Joseph B. Stearns of Boston, Massachusetts (United States) invented the first working FDX communications circuit, which was installed in 1872 on a onewire telegraph system using a ground return. This system effectively doubled the traffic capacity of the circuit, and at much lower cost than stringing another wire. See also HDX and simplex.