Early PCs used the AT form factor and 12in wide motherboards. The sheer size of an AT motherboard caused problems for upgrading PCs and did not allow use of the increasingly popular slimline desktop cases. These problems were largely addressed by the smaller version of the full AT form factor, the Baby AT, introduced in 1989. Whilst this remains a common form factor, there have been several improvements since.

All designs are open standards and as such don’t require certification. A consequence is that there can be some quite wide variation in design detail between different manufacturers\’ motherboards. However, keeping to the standards allows case manufacturers to develop for particular motherboard form factors, a boon for home builders, modders and PC technicians.

The table below shows common motherboard form factors in use.

Table of Common Form Factors

Name PCB Size (mm)
AT 350×305
ATX 305×244
Baby-AT 330×216
BTX 325×266
COM Express 125×95
DTX 244×203
EBX 203×146
EPIC (Express) 165×115
ESMexpress 125×95
ETX / XTX 114×95
FlexATX 229×191
LPX 330×229
microATX 244×244
microATX (Min.) 171×171
Mini-DTX 203×170
Mini-ITX 170×170
mobile-ITX 75×45
Nano-ITX 120×120
NLX 254×228
PC/104 (-Plus) 96×90
Pico-ITX 100×72
WTX 356×425

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