1. [n]  an apartment having rooms on two floors that are connected by a staircase
2. [n]  a house with two units sharing a common wall
3. [adj]  (telecommunication) allowing communication in opposite directions simultaneously; "duplex system"; "duplex telephony"
4. [adj]  (used technically of a device or process) having two parts; "a duplex transaction"

1. \Du"plex\, a. [L., fr. duo two + plicare to fold. See
   {Two}, and {Complex}.]
   Double; twofold.
   
   {Duplex escapement}, a peculiar kind of watch escapement, in
      which the scape-wheel has two sets of teeth. See
      {Escapement}.
   
   {Duplex lathe}, one for turning off, screwing, and surfacing,
      by means of two cutting tools, on opposite sides of the
      piece operated upon.
   
   {Duplex pumping engine}, a steam pump in which two steam
      cylinders are placed side by side, one operating the
      valves of the other.
   
   {Duplex querela} [L., double complaint] (Eccl. Law), a
      complaint in the nature of an appeal from the ordinary to
      his immediate superior, as from a bishop to an archbishop.
      --Mozley & W.
   
   {Duplex telegraphy}, a system of telegraphy for sending two
      messages over the same wire simultaneously.
   
   {Duplex watch}, one with a duplex escapement.
   
   

2. \Du"plex\, v. t. [See {Duplex}, a.] (Teleg.)
   To arrange, as a telegraph line, so that two messages may be
   transmitted simultaneously; to equip with a duplex
   telegraphic outfit.
   
   

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.

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