A circuit that consists of two conductors insulated from each other and that provides simultaneously a two-way communication channel in the same frequency band between two points in a telecommunication system. The circuit may be a phantom circuit in which case it is formed from two groups of conductors. A circuit that operates in the same manner but that is not limited to only two conductors (or groups of conductors) is termed a two-wire type circuit. See also transmission line; four-wire circuit.