Editing Telephone jack and plug (section)

==History==
Historically, telephones were typically owned by the telephone company, and were usually permanently wired to the [[telephone line]]. 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. Before {{Circa|1930}}, concentric connectors with three contacts were usually sufficient, but the upgrade of telephone sets to anti-[[sidetone]] circuitry in the 1930s required at least four conductors between a desk set and the [[subscriber set]] that contained the [[telephone hybrid]] and a [[Ringing (telephony)|ringer]]. For this purpose, Bell System engineers developed a cube-shaped four-prong plug (type no. 283) with uneven prong spacings to avoid improper insertion into the jack.<ref name=blr1949>{{cite journal |title=Jacks and Plugs for Portable Telephones |journal=[[Bell Laboratories Record]] |last=Krantz |first=H.K. |date=July 1944 |volume=12 |issue=7 |pages=343–346 |url=https://worldradiohistory.com/Archive-Bell-Laboratories-Record/30s/Bell-Laboratories-Record-1934-07.pdf#page=22 |access-date=October 28, 2022}}</ref><ref>AT&T, ''Jacks and Plugs'', Bell System Practices, [http://etler.com/docs/BSP/C/C36/C36.201_I1.pdf Section C36.201 Issue 1] (February 1, 1932)</ref>

The cubic design was changed to a round version (No. 505A) in the mid 1960s. The four-prong jack and plug combination was the standard line connection for all portable telephone sets until the conversion to [[modular jack]]s in the 1970s, typified by the [[Registered Jack]] standards promulgated in U.S. federal law.

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.

<gallery>
file:Western Electric 404A and 283B four-prong telephone jack and plug combination ca1960.jpg|Bell System Type 404A jack and type 283B plug ({{Circa|1960}})
Image:4 prong plug jeh.jpg|Trimline telephone line cord with a Type 505A wall plug and modular set plug (mid-1960s)
File:Four-prong telephone jack, 1960s, front.jpg|The front of a US four-pronged telephone jack (1964)
File:Four-prong telephone jack, 1960s, back.jpg|The back of a US four-pronged telephone jack (1964)
</gallery>