Used to describe a communications channel that can carry SIgnals in both directions, in contrast to a SImplex channel which only ever carries a SIgnal in one direction. If SIgnals can only flow in one direction at a time the communications is "half-duplex", like a SIngle-lane road with traffic lights at each end. Walkie-talkies with a "press-to-talk" button provide half-duplex communications. If SIgnals can flow in both directions SImultaneously the communications is "full-duplex", like a normal two-lane road. Telephones provide full-duplex communications. The term "duplex" was first used in wireless, telegraph, and telephone communications. Nearly all communications circuits used by computers are two-way, so the term is seldom used. . (2001-07-21)