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RJ9 is the telecommunications jack or cable connectors you use to connect your phone’s handset to the telephone base unit. It looks like a smaller version of an RJ11. Usually, these cables are curly.

RJ9 (Wikipedia)

Left to right, modular connectors: 8P8C plug, 6P6C plug, 6P4C plug, 4P4C plug, 6P6C jack.
An 8P8C modular plug. This is the common crimp type plug, of the same kind pictured above crimped onto a cable (with molded sleeve).

A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such in computer networking, telecommunication equipment, and audio headsets.

Modular connectors were originally developed for use on specific Bell System telephone sets in the 1960s, and similar types found use for simple interconnection of customer-provided telephone subscriber premises equipment to the telephone network. The Federal Communications Commission (FCC) mandated in 1976 an interface registration system, in which they became known as registered jacks. The convenience of prior existence for designers and ease of use led to a proliferation of modular connectors for many other applications. Many applications that originally used a bulkier, more expensive connector have converted to modular connectors. Probably the best known applications of modular connectors are for telephone and Ethernet.

Accordingly, various electronic interface specifications exist for applications using modular connectors, which prescribe physical characteristics and assign electrical signals to their contacts.

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