ATX

Designed and released in 1995, Intel ATX is a new motherboard form factor with a physical design like the traditional board (30.5 cm X 19 cm) shifted 90 degrees for better placing of the units. The I/O connectors COM1, COM2 and LPT, keyboard, mouse and USB are mounted directly on the motherboard. The ATX board requires specifically designed chassis with an I/O access opening measuring 1 by 6 inch. 

The ATX motherboard includes advanced control facilities, where the BIOS program continually checks the CPU temperature and voltages, the cooling fans RPM, etc. If over heating occurs, the PC will shut down automatically. The PC can also be turned on by, for example, modem signals, since the power supply is controlled by the main board. The on/off button will turn the PC "down" without turning it completely off. The computer will also not be able to be turned off while the computer boots up; however, if the computer freezes as it is turning on you can turn off the computer by holding the power button for 5 seconds.

Also see: BTX, Form factor, Motherboard definitions