title: Four-wire terminating set
text: # Four-wire terminating set
  
  A four-wire terminating set (4WTS) is a balanced transformer used to perform a conversion between four-wire and two-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.
  
  The 4WTS differs from a simple hybrid coil in being equipped to adjust its impedance to maximize return loss.
  
  Four-wire terminating sets were largely supplanted by resistance hybrids in the late 20th century.
  
  