In the T568 wiring, sometimes called the RJ-45, pin 3 and pin 6 have the same color pair. I've wondered why it was so decided. I think it would have been simpler to make the 3, 4 and 5, 6 pins the same color pair. Is there any reason?
It's done that way because the 6p connector (standard telephone wiring, with either two wires:== RJ11 or four wires:==RJ14) puts TIP and RING on the center two positions, with TIP2 and RING2 (for a second phone line) flanking them.
The prime pair in the six-position plug is pins #3 of 6 and 4 of 6, and secondary pair is pins 2 and 5. Going to an eight-position connector for Ethernet, the scheme was kept for those four center wires, now renumbered 3-4-5-6 and new pairs were put on the 1-2 of 8 and 7-8 of 8 positions.
There was a long-ish period, with 10baseT and 100baseT wiring, which only used four wires, and some (annoying) cable systems only routed four wires for Ethernet; those weren't compatible with 1000baseT when gigabit Ethernet was introduced. The four-wire cables were using pins 1-2 and 3-6 of the eight-wire connector, so would allow a 4-5 pair to be added for telephones...