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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''. ==History== 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 mobile 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 1930 concentric connectors with three contacts were sufficient. The most common connector type in the Bell System in the United States was developed by 1930. It was a cube-shaped four-prong connector type (No. 238) with uneven prong spacings to prevent improper insertion of the plug into the jack (type 404). This type was redesigned as a round version (No. 505A) in the mid 1960s. The four-prong connector type was used for several decades until it was superseded by the modular connector in the 1970s.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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「telephone plug」の詳細全文を読む スポンサード リンク
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