The difference between an RJ11 and an RJ12 cable is based on the number of wires used within the cable and how the wires are arranged. RJ11 includes four wires, and two slots are not used. RJ12 uses wires in all six available slots.
"RJ" refers to the standard for registered jacks used in telephone lines. The abbreviation includes two numbers identifying the specific purpose for the connector. The RJ11 and RJ12 standards look very similar because they use the same connector, which includes six slots.
Telephone lines use both wiring standards, but RJ12 is additionally used in some other applications, such as corporate private branch exchange phone systems. RJ12 is used less commonly than is RJ11, which is used frequently in home telephone service. RJ11 also is used in digital subscriber lines to provide broadband Internet access and for other purposes.
RJ refers to both the female connector and its accompanying wiring, but confusion of the term's meaning leads to its misuse as a general term for connectors, such as those used for Ethernet. The Bell System introduced registered jacks in 1976. The Federal Communications Commission adopted the new standard as federal law in the 1980s. In addition to RJ11 and RJ12, several other types of RJ lines are used in alarm systems and data connections.