. . though ATX was also available. Just look on the back of the cabinet. If there's a removable plate including the keyboard, mouse, serial, parallel etc. ports, it's ATX. If there is no insert plate or there is one with only the keyboard and all ports are in slots, it's AT.

Of course, there's probably some mixed ones. The AT motherboard we used for the last year we used AT accepted either AT or ATX power supplies.

With ATX, the power on/off is handled by the motherboard and the switch is momentary. With AT, power on/off is handled by a real on/off switch. Microsoft found this unacceptable because Windows didn't have control over power on/off.