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25 Pair to RJ45

Telephone pairs are designated Tip and Ring (i.e. Tip as in Tip of the plug they used to use in the old switch boards). They are always paired with the fist wire being tip and positive, and the second ring and negative.

In larger bundles there are Major and Minor colors. 

Major colors are Tip and minors are Ring

In a RJ45, Blue is Ring, Blue-White is Tip.

So major colors go to the blue-white ping in an RJ45