In telecommunication, a hybrid coil (bridge transformer) is a single transformer that effectively has three windings, and which is designed to be configured as a circuit having four branches, (i.e. ports) that are conjugate in pairs.

• The primary use of a hybrid coil is to convert between 2-wire and 4-wire operation in concatenated sections of a communications circuit. Such conversion is necessary when repeaters are introduced in a 2-wire circuit.

Source: from Federal Standard 1037C and from MIL-STD-188