The motherboard is objectively the most frustratingly complicated piece of computer hardware to figure out. Show me an entire GPU generation line-up and I can figure out what I want for my budget in very little time at all. Show me two motherboards and I'll be umming and ahing for quite some time. At least, that was the case, but now some absolute legend has made a tool that should make things a lot easier.
The web-based tool, MoboMaps by Robin Wilde, does what I think most people who have been in the gaming PC building bizz for a while will recognise as a giant service to humanity. It shows you visually and interactively exactly where different motherboards have different PCIe lanes routed.
