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RJ (Registered Jack) connectors are plug-in connectors for computer network and telecommunications cabling. They are standardized according to the Code of Federal Regulations Part 68. The standard contains both the structure of plugs and sockets and the contact assignment.

The term RJ connectors was coined by Bell Laboratories in the 1970s. The connectors quickly gained popularity and were standardized by the Federal Communications Commission shortly thereafter.

The RJ-45 is an 8P8C (8 Position 8 Contatcs) twisted pair pinout, which is used especially in Ethernet and ISDN networks. It can be found especially in

• Junction boxes and connecting cables
• Structured cabling
• Computer networks

The connectors can have four, six or eight poles. However, regardless of the number of poles, they all fit into the eight-pole socket. In each case, the two inner contacts formed a pair, the next outer, and so on to the outermost contacts. However, since this concept was not suitable for high-speed LAN protocols, two pinout variants were standardized under the TIA-568A/B designation. The reason for this is that the two outermost wires are too far apart.

With TIA-568A/B, two adjacent contacts form the third and fourth pairs. The A and B standard can be distinguished by the color coding. Only the wire pairs 2 and 3 are interchanged.

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