A phone socket is a jack that can be used to connect phones and other modular equipment to the local telephone network. They are designed for the easy installation and removal of new equipment, which can be contrasted with the hard wired phones that were common throughout the world until the 1970s. The standard used in the United States and many other countries is the registered jack (RJ) system, which actually refers to a wiring pattern in addition to the physical plugs. A handful of other phone sockets are also used around the world, such as the BS (British standard) 6312 jack in the UK and the Telekommunikations-Anschluss-Einheit (TAE) connector in Germany.
From the time telephones were first introduced until the 1970s, phone companies tended to own all of the hardware. This included the wiring inside homes and business and even the phones, which were leased to the customers. Phones tended to be hard wired during this period, and even when some type of phone socket was used it was typically a proprietary design. The first phone socket standards were created in the 1970s, which is also when third-party phones that could make use of these modular connections were introduced.
The most common type of phone socket is the registered jack, which is a standard that describes both a wiring pattern and socket design. RJ11 is the type of phone socket typically used for a single phone line, while RJ12 and RJ45 use the same type of jack but are wired for two or three lines. These sockets originated in the United States, though they are also prevalent throughout the rest of the world.
Some countries have their own types of phone jacks, such as the BS 6312 socket in the UK. These sockets were introduced in the early 1980s and bear a superficial resemblance to RJ connectors. The retainer clips are on the side instead of the bottom though, which makes them easy to tell apart. BS 6312 sockets are also used in some other countries, though there have been efforts to phase them out in locations such as Hong Kong.
In addition to telephones, various other devices can be plugged into phone jacks. Fax machines can communicate over phone lines and are designed to be plugged into these sockets. Dial-up modems also use these jacks, as do broadband digital subscriber line (DSL) connections. The modular nature of phone jacks and plugs makes it relatively easy to connect a wide variety of compatible devices.