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.