RJ stands for Registered Jack, part of the Universal Service Ordering Code system developed by Bell Systems and adopted in part by the FCC. The Registered Jack wiring configurations allow communication between the consumer's premises and the public network. The pinout for a single line-6 contact male connector is contact 3-red and contact 4-green using the Bell color system.
The pinout for 2 lines is contact 2-black, contact 3-red, contact 4-green and contact 5-yellow and is referred to as RJ14. An RJ25 male connector pinout for 3 lines is contact 1-white, contact 2-black, contact 3-red, contact 4-green, contact 5-yellow and contact 6-blue. The RJ11, RJ14 and RJ25 all use the same physical 6 contact male connector. All phone wiring comes in pairs. The red-green pair (3 and 4) make up the single phone line, while the black and yellow (2 and 5) comprises the second line. The white and blue (1 and 6) are the third line pair.
There are other color schemes for wiring an RJ11, RJ14 and RJ25. The German color scheme on contacts 1-6 are violet, green, white, brown, yellow and slate. The Australian colors are orange, red, blue, white, black and green. For a 25 pair cable where the wires are two colors, from 1-6 is white/green, white/orange, blue/white, white/blue, orange/white and green/white.