AT (form factor)
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In the era of IBM compatible personal computers, the AT form factor comprises the dimensions and layout of the motherboard for the IBM AT. Baby AT motherboards are slightly smaller, measuring 8.5" by
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
ATX
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ATX is a motherboard and power supply form factor and configuration specification developed by Intel. It aimed to improve on previous de facto standards such as the AT design. Originally released in J
AT (form factor)
…contributing to its popularity. In the 1990s many computers still used AT and its variants. Since 1997, the AT form factor has been largely supplanted by ATX. 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…
An ATX motherboard
Comparison of some common motherboard form factors (pen for scale)
ATX I/O plates for motherboard rear connectors
24-pin ATX motherboard power plug; pins 11, 12, 23 and 24 form a detachable separate four-pin plug, making it backward-compatible with 20-pin ATX receptacles