In data communications, duplex refers to whether the communication can happen in only one direction at a time, or in both directions at once. Half-duplex means communication can only occur in one direction at at time, like a CB radio; full-duplex means communications can occur in both directions at once, like a telephone. Old modems used to have a mechanical switch to specify half- or full-duplex, so a common cry over a telephone line when attempting to get a data call going was "Check your duplex switch!"