Telephone jack and plug

The plug is inserted into its counterpart, the jack, which is commonly affixed to a wall or baseboard.

For some applications 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 in various designs with various numbers of contacts.

For this purpose, Bell System engineers developed a cube-shaped four-prong plug (type no.

They are also sometimes used to provide power for telephone dial lamps (6 volts AC, as in the Princess phone), or other features.

This list covers only single-line telephone plugs commonly used in homes and other small installations; there are 44 different variations of plugs, including an Israeli version of BS6312 with different internal wiring of the pins, plus hard wiring to a junction box with no adapter.