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Modular Jacks & Plugs		Jacks and plugs are wired to conform to Uniform Service Ordering Code ("USOC") numbers, originally developed by the Bell System, and endorsed by the FCC. One specific piece of hardware can be wired in different ways, and have different USOC numbers. USOC has become an acronym, pronounced "you-sock," and jack wiring schemes are generally referred to as "USOC codes."

The 8-position modular jack (above) is commonly and incorrectly referred to as “RJ45”. The 6-position modular jack is commonly referred to as RJ11, which may or may not be correct. Using RJ terms often leads to confusion since the RJ designations refer to very specific USOC wiring configurations. The designation ‘RJ’ means Registered Jack, and should be used only for jacks that are connected directly to phone company circuits.

Each of the basic jack styles can be wired for different RJ configurations. For example, the 6-position jack can be wired as an RJ11C (1-pair), RJ14C (2-pair), or RJ25C (3-pair) configuration. An 8-position jack can be wired for configurations such as RJ61C (4-pair) and RJ48C. The keyed 8-position jack can be wired for RJ45S, RJ46S, and RJ47S. The fourth modular jack style is a modified version of the 6-position jack (modified modular jack or MMJ). It was designed by Digital Equipment Corporation (DEC) along with the modified modular plug (MMP) to eliminate the possibility of connecting DEC data equipment to voice lines and vice versa. (This paragraph is based on info from Siemon, a supplier of much of the telecom hardware we use and sell.)

In the diagrams below, you are looking into the opening of the jack. Internal wiring in the jack may have different colors from the wiring that goes to the jack.