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A series of connectors generally used for phone wires, networking, or anything else using twisted pair copper wires. Typically they are made out of clear plastic. They have six sides. The wire trails out the back. The top includes a small tab to lock it in place in a female connector. The front/bottom corner and bottom side contains 4,6 or 8 gold fingers depressed in the plastic, running front to back. These make contact with the female connector's wires when put in place. Any RJ connector may fit into a female connector of a bigger size and still be held in place. They are listed below in decreasing size order (it is the width that varies between connectors).

RJ-45 - mainly networking
RJ-11 - mainly phone wires
RJ-12 - mainly cheap phone wires