T568A and T568B are the two color codes used in wiring RJ45 eight-position modular plugs. The American National Standard Institute/Telephone Industry Association/Electronics Industry Association (ANSI/TIA/EIA) wiring standards allow both of these color codes. The only difference is that the orange and green pairs are interchanged.
Because it provides backward compatibility for both one pair and two pair Universal Service Order codes (AT&T) USOC wiring schemes, the T568A wiring pattern is recognized as the preferred wiring pattern for this standard.
However, the T568B standard (used by VPI) is the most widely used wiring scheme, as it matches the older ATA&T 258A color code. It is also permitted by the ANSI/TIA/EIA standard, but it provides only a single pair backward compatibility to the USOC wiring scheme.
U.S. Government regulations require the use of the preferred T568A standard for wiring done under federal contracts.
The following diagrams look at the jacks from the front. The wiring at the rear of the jack varies by manufacturer; it may not be the same sequence as the front. Compliance with the color codes is maintained by routing the connections at the back to the proper sequence at the front of the jack. This is done by a small printed-circuit board in the jack assembly. Cat 5e jacks (diagram below right) may have a twist inside the jack to reduce crosstalk.
