The predominant motherboard form factor since the mid-1990s. It improves on the previous standard, the Baby AT form factor, by rotating the orientation of the board 90 degrees. This allows for a more efficient design, with disk drive cable connectors nearer to the drive bays and the CPU closer to the power supply and cooling fan.
Intel's ATX motherboard specification improved on the old AT spec by rotating the whole arrangement 90 degrees in the case, and moving the CPU and RAM away from the expansion slots so long cards wouldn't foul them. ATX also supports a smarter power supply, which can turn on and off in response to port activity and software control. Computers that can turn themselves off when you tell them to shut down use ATX power supplies. Full size ATX boards are 305mm by 244mm. Mini-ATX boards are 284mm by 208mm. Both should fit in any ATX case.
A standard that defines the number and position of the motherboard mounting holes and offers recommendations as to component, expansion board, and port connector placement. An ATX board must use an ATX Power Supply.