AT (form factor)

The original AT motherboard, later known as "Full AT", is 13.8 × 12 inches (351 × 305 mm), which means it will not fit in "mini desktop" or "minitower cases".

(In IBM's original heavy-gauge steel case, the two 5+1⁄4" full-height drive bays overhang the front of the motherboard.

In 1987, the Baby AT form factor was introduced, based on the motherboard found in the IBM PC/XT 286 (5162)[2] and soon after all computer makers abandoned AT for the cheaper and smaller Baby AT form factor, using it for computers that spanned several generations, from those that used 286 processors to the P5 Pentium and a limited number of Pentium II systems.

The development of bigger CPU coolers—and the fact that they blocked full-length PCI and ISA cards—spelled the end of Baby AT and was the main impetus for its successor ATX.

In 1995, Intel introduced ATX, a form factor which gradually replaced older Baby AT motherboards.

IBM PC AT System Board. This is the original AT motherboard on which the form factor was based.
Keyed AT-style power connectors to prevent improper insertion
Baby AT motherboard
An ATX Form Card, used by later Baby-AT motherboards to allow for USB, PS/2 mouse , and IR connectivity through headers