USOC. A wiring standard for telephone service to an individual phone. In a modular plug, the two center conductors are the primary pair, and supplementary pairs radiate out concentrically. For example, in an RJ-45 (eight pin) plug, pins 4,5 are the primary pair with pins 3,6 - 2,7 - 1,8 forming the supplementary pairs. Note that this is not compatible with TIA/EIA 568-A's recommendation of T-568A or T-568B.