This isn't true, but it might happen sometime in the future. Say that you have a whole pc built and functional. You want to change your processor (type A) to something your old motherboard doesn't support. So you get a new motherboard that supports your new CPU (type B) and all your old parts. You install the 2 new parts (mobo and cpu) and nothing runs. Doesn't start up. Nothing. So, you know that it is either your CPU or your motherboard. But - how do you know exactly whether it is one or the other or both? Especially if you don't have a spare processor of the new type (type B). (This might happen when I change my motherboard and CPU to a conroe setup, reusing my ram, hdd, gfx card, dvd, floppy, case, and powersupply).