A four-wire terminating set is a balanced transformer used to perform a conversion between 4-wire and 2-wire operation in telecommunication systems. 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 circuit into a 2-wire circuit, in which loop repeaters may be situated to amplify signals in each direction without positive feedback and oscillation. Four-wire terminating sets were largely supplanted by resistance hybrids in the late 20th century.