History
Registered Jacks were introduced by the Bell Labs in the 1970s, replacing much bulkier connectors that were in use before. Bell Labs gave the specifications for them (both the modular connectors and the wiring of them) Universal Service Ordering Codes (USOC), and that was the only standard at that time. AT&T and the Bell System used its leasing arrangements for telephones and telephone equipment made by its subsidiary, Western Electric to increase its control over telephone manufacturing in the United States and Canada. Western Electric started to use the Modular Connector, therefore the name Western Plug was born. When the US telephone industry was opened to more competition in the 1980s, the specifications were made a matter of US law, ordered by the Federal Communications Commission (FCC) and codified in the Code of Federal Regulations, 47 CFR 68, subpart F. In January 2001, the FCC turned over responsibility for standardizing connections to the telephone network to a new private industry organization, the Administrative Council for Terminal Attachment (ACTA). The FCC removed Subpart F from the CFR and added Subpart G, which delegates the task to the ACTA. The ACTA published a standard called TIA/EIA-IS-968 which contained the information that was formerly in the CFR. The current version of that standard, called TIA-968-A, specifies the modular connectors at length, but not the wiring. Instead, TIA-968-A incorporates a standard called T1.TR5-1999 by reference to specify the wiring. Note that a Registered Jack name such as RJ11 identifies both the physical connectors and the wiring (pinout) of it.

Common usage
The most familiar registered jacks are the 6-position connectors known variously as RJ11 (2 conductor/1 pair), RJ12 or RJ25 (6 conductor/3 pair), and RJ14 (4 conductor/2 pair); and the RJ45 (8 conductor/4 pair), all sometimes simply called RJ connectors or modular phone plugs/jacks. These are commonly used in building wiring for telephone and local area networks. They were originally invented and patented by Bell Telephone Laboratories (patent filed 6 July 1973; U.S. Patent 3,860,316 issued 14 January 1975), and replaced the hard-wired connections on most Western Electric telephones around 1976. Thus, they are also sometimes called Western jacks and Western plugs.

Naming confusion
There is much confusion over these connection standards. The six-position plug and jack commonly used for telephone line connections may be called an RJ11, RJ14 or even RJ12 (RJ25), all of which are actually names of interfaces that use this physical connector. The RJ11 standard dictates a 2-wire connection, while RJ14 spells out a 4-wire configuration, and RJ12 or RJ25 uses all six wires. Plugs and jacks of this type are often called modular connectors, which originally distinguished them from older telephone connectors, which were very bulky or wired directly to the wall and therefore not accommodating of modular systems. A common nomenclature for modular connectors is e.g. "6P" to indicate a six-position modular plug or jack. Sometimes the nomenclature is expanded to indicate the number of positions that contain conductors. For example, a six-position modular plug with conductors in the middle two positions and the other four positions unused is called a 6P2C. RJ11 uses a 6P plug; furthermore, it often uses a 6P2C. An alternative terminology sometime used is e.g. 6x2. Another alternative is e.g. 6/2