A communications system where the path is different for talk and listen. In intercom channels, there are four wires (two paths). Four-wire systems can be four-wire balanced or four-wire unbalanced. Four-wire audio is more of less defined as a pair of conductors carrying an input/receive signal and a second pair carrying the output/send signal. The four-wire circuit gets its name from the fact that a balanced pair of conductors was used in each of two directions for full-duplex operation.