In the early days of the computer, the AT and baby AT form factors were the most common motherboard form factors. These two variants differ primarily in width: the older full AT board is 12″ wide. It is an obsolete motherboard form factor only found in older machines, 386 class or earlier.
One of the major problems with the width of this board (aside from limiting its use in smaller cases) is that a good percentage of the board “overlaps” with the drive bays. This makes installation, troubleshooting and upgrading more difficult.
A Baby AT motherboard is 8.5″ wide and 13″ long. The reduced width means much less overlap in most cases with the drive bays, although there usually is still some overlap at the front of the case.
Baby AT motherboards are distinguished by their shape, and usually by the presence of a single, full-sized keyboard connector soldered onto the board. The serial and parallel port connectors are almost always attached using cables (ribbons) that go between the physical connectors mounted on the case, and pin “headers” located on the motherboard. Most of the boards use AT power supplies and the system units tend to be tower casing.