Telephone plug

A telephone plug is a type of connector used to connect a telephone set to the telephone wiring inside a building, establishing a connection to a telephone network. It is inserted into its counterpart, a telephone jack, commonly affixed to a wall or baseboard. The standard for telephone plugs varies from country to country, though the RJ11 modular connector has become by far the most common.

A connection standard, such as RJ11, specifies not only the physical aspects of an electrical connector, but also the pinout, i.e. the assignment or function of each contact. Modular connectors are specified for the registered jack (RJ) series of connectors, as well as for Ethernet and other connectors, such as 4P4C (4 position, 4 contacts) modular connectors, the de facto standard on handset cords, often improperly referred to as RJ connectors.

Historically telephones were typically owned by the telephone company and were usually permanently wired to the telephone line. However, for many installations it was necessary or convenient to provide portable telephone sets that could be moved to a different location within the customer's premises. For this purpose telephone companies developed jacks and plugs with a varying number of contacts. Before ca. 1930, concentric connectors with three contacts were sufficient, but the upgrade of telephone sets to anti-sidetone circuitry required four conductors between the desk set and the subscriber set. For this purpose, Bell System engineers developed a cube-shaped four-prong plug (type No. 238) with uneven prong spacings to avoid improper insertion into the jack (type No. 404). This jack and plug combination later became the standard line connection for portable telephone sets.

This type was redesigned as a round version (No. 505A) in the mid 1960s. The four-prong connector type was superseded by the modular connector in the 1970s.

Many countries initially used different specifications for connectors, and some national connector types remain in service, but few are used for new installations for which modular connector types are prescribed.

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