Spend an hour crimping cable ends, plug everything in, and... nothing works. You check the connectors—they look fine. Then you compare both ends and realize you mixed up the color order. Now you're pulling the cable out and starting over.
Inside every Cat 5 cable, eight copper wires follow a color system that's been around since the 1990s. Match these colors correctly, and your network transmits data flawlessly. Mix them up, and you'll chase phantom connection issues for hours.
Back in 1991, the Telecommunications Industry Association introduced two wiring schemes: T568A and T568B. Both work perfectly fine—you just need to pick one and stick with it across your entire network.
Here's what matters: each cable contains four pairs of twisted wires. One wire in each pair has solid color insulation (orange, green, blue, brown). Its partner wire is mostly white with a stripe matching that solid color. So you get white-with-orange-stripe paired with solid orange, white-with-green-stripe paired with solid green, and so on.
T568A puts the green pair on positions 1 and 2 at the connector. T568B flips things around—orange takes those first two spots instead. Everything else? Pretty much the same between both standards.
Walk into most office buildings in the US, and you'll find T568B everywhere. Government facilities often use T568A (some federal specs actually require it). Homes? Complete mix. I've seen houses wired with T568A, houses with T568B, and ...