A circuit that consists of two pairs of conductors and that forms a simultaneous two-way communication channel between two points of a telecommunication system. One pair of conductors forms the ‘go’ channel and the other the ‘return’ channel. In the case of a two pairs or groups of conductors form the circuit. A circuit that operates in the same manner as a four-wire circuit, although not necessarily containing four conductors, is a four-wire type circuit. An example is a circuit that contains two wires and that has a different frequency band for each direction of transmission. See also transmission line; two-wire circuit.