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. Late in the 20th century, almost all connections between telephone exchanges were four-wire circuits, while conventional phone lines into residences and businesses were two-wire circuits.
The four-wire circuit gets its name from the fact that, historically, a balanced pair of conductors were used in each of two directions for full-duplex operation. The name may still be applied to, for example, optical fibers, even though only one fiber is required for transmission in each direction. A system can separate the frequency directions by frequency duplex and realize the benefits of a four-wire circuit even while the same wire pair is used in both directions.