The AT & Baby AT Form Factors
Form factor design has had an interesting history. IBM designed the Advanced Technology (AT) form factor in 1984. Its dimensions were 12 × 11–13 in or (305 × 279–330 mm). The IBM PC was introduced in 1981, but the vaunted AT model was released three years later.
The rise of the motherboard's defacto importance continued as more chips and circuits were added to it, correspondingly making the motherboard area smaller. Problems with the AT form factor were due principally with the physical size of the board. At 12" wide, it would cause the board to overlap with space required for the drive bays. The Baby AT form factor was released in 1985 by IBM, with dimensions 8.5 × 10–13 in (216 × 254–330 mm). Clearly, the size reduction made the overlap problem disappear.
The following image breaks down the different components of a motherboard.
While the size of the Baby AT solved some problems, it also created others. The Baby AT form factor did not provide the space to use a combination of processor, fan and heat sink. To solve these space issues, a new form factor design took place, namely the ATX.
The Baby AT and AT boards have similar dimensions, but the ATX board has a 90 degrees rotation within the case to allow for easier access to components.
Other information about form factors is available at What is a Motherboard Form Factor?