An open PC motherboard specification by Intel.
ATX is a development of the Baby AT specification with the motherboard rotated 90 degrees in the chassis. The CPU and SIMM sockets have been relocated away from the expansion card slots meaning that all the slots support full-length cards. More I/O functions are integrated on the motherboard. As the longer edge of the board is now at the back of the chassis, there is more space for connectors; also, the I/O opening on the back panel of the chassis has been defined as double the previous height, allowing vendors to add extra on-board I/O functions over and above the standard.
Most Pentium Pro boards use this form factor.
As well as the motherboard size, layout, and placement, the ATX specification also includes requirements for power supply and fan specification and location.
The full size ATX board measures 305mm wide by 244mm deep. There is also a Mini-ATX form factor, 284mm by 208mm.