Four-wire circuit

In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path.

Note: The four-wire circuit gets its name from the fact that, historically, two conductors were used in each of two directions for full-duplex operation[?]. The name may still be applied, e.g. , to a communications link supported by optical fibers, even though only one fiber is required for transmission in each direction.

Contrast with two-wire circuit[?].

Source: from Federal Standard 1037C and from MIL-STD-188