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)