What is an AT motherboard?

Answer 1

An AT motherboard is a motherboard that has the same form factor as the IBM PC/AT. This form factor was popular from about 1984 to 1995. An AT motherboard measures approximately 350×305 mm.

To be specific, the IBM AT was the first 80286-based computer. It's case and motherboard were a bit larger than the IBM XT computer, and unlike the XT, it used CMOS, a battery for the CMOS, and a separate software tool to set the CMOS settings. Most of the PC clones followed the AT standards in terms of the case and mounting dimensions, what was available on the back of the computer, the size of the keyboard plug, the keyboard's signals, the way the power supply turned on, and the presence of a CMOS chip for the settings.

Then by the mid 1990's the ATX standard emerged. That made a number of changes. It added a rectangular slot to allow for different things being on the motherboard with the use of appropriate I/O plates, allowing manufacturers to rely on expansion boards less. It used a low voltage signal that was always present to power on the computer, changed the motherboard power connectors, used a PS/2 style keyboard, and made perhaps a few other changes.