A transmission circuit composed of two wires - signal and ground used to both send and receive information. In contrast, a four-wire circuit consists of two pairs. One pair is used to send. One pair is used to receive. All trunk circuits - long distance circuits - are four wire. A four-wire circuit costs more but delivers better reception, All local loop circuits - those coming from a Class 5 central office to the subscriber's phone system - are two wire, unless you ask for a four-wire circuit and pay a little more, Twisted-pair is a widely-used type of two-wire circuit. See also Twisted Pair.