Registered Jack:
https://en.wikipedia.org/wiki/Registered_jack#RJ45S
...
https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair
100BASE-TX uses only two pairs
using the pairs at pin-positions 1&2 and 3&6.
Straight-through, Crossover, and Rollover Wiring
https://www.computercablestore.com/straight-through-crossover-and-rollover-wiring