In telecommunication, a four-wire terminating set is a balanced transformer used to perform a conversion between 4-wire and 2-wire operation.
Note 1: For example, a 4-wire circuit may, by means of a 4-wire terminating set, be connected to a 2-wire telephone set. Also, a pair of 4-wire terminating sets may be used to introduce an intermediate 4-wire loop into a 2-wire circuit, in which loop repeaters may be situated to amplify signals in each direction without positive feedback and oscillation.
Note 2 : Four-wire terminating sets have been largely supplanted by resistance hybrids.
Source: from Federal Standard 1037C and from MIL-STD-188
