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Phone jacks and plugs are connectors used with telephones and other audio systems. They are also known as modular jacks (female) and modular plugs (male). The term “modular” describes a telephone connection interface developed by AT&T that allows installers to assemble phones at customer locations by using components that can be plugged together instead of hard-wired. Phone jacks are female receptacles into which plugs or male connectors are inserted. Several types of phone jacks and plugs are commonly available. In the United States, telephone jacks are called registered jacks (RJ), licensed with the Federal Communications Commission (FCC), and described as RJ-nn, where nn is a two-digit number. The RJ-11, the most common type of telephone jack, can have six conductors but is usually implemented with four. Typically, these four wires consist of a red and green pair that carries voice and data, and a black and white pair that is used for low-voltage signals. Computers that use dial-up modems to connect to networks use mainly RJ-11 jacks. The RJ-45, another common type of jack, is a single-line device for digital transmissions over twisted or untwisted telephone wires. Both keyed and unkeyed versions of the RJ-45 are available. Other types of phone jacks and plugs include the 1/4” jack and the 1/8” jack, both of which are used as connectors in audio systems, amplifiers, and speakers. The 1/4” jack has a 0.25” diameter. The 1/8” jack has a 0.125” diameter.

Phone jacks and plugs are housed in metal or plastic cases and differ in terms of mounting styles and performance specifications. Typically, wall or box mounted devices are flanged, threaded for an accessory such as backshell, and designed for permanent mounting. Connectors with receptacle-style mounting mechanisms are used to fit panel cutouts. Dummy receptacles do not have provisions for attaching conductors and are generally used for the storage of a cable assembly connector plug. Performance specifications for phone jacks and plugs include voltage rating, current rating, contact resistance, insulation resistance, dielectric withstanding voltage, and operating temperature. Contact resistance is the measurement of the electrical resistance of mated contacts when assembled in a connector under typical service use. Electrical resistance is determined by measuring from the rear of one contact area to the rear of the contact area of its mate (excluding both crimps) while carrying a specified test current. Dielectric withstanding voltage is the maximum potential gradient that a dielectric material can withstand without failure. The values that suppliers provide depend upon the thickness of the material and the method and conditions of the test. Another important product specification for phone jacks and plugs is the number of contacts or ways that mate with corresponding elements to provide an electrical path.