Up until recently, the AT and baby AT form factors were the most common form
factor in the motherboard world. These two variants differ primarily in width: the older
full AT board is 12" wide. This means it won't typically fit into the commonly used "mini"
desktop or minitower cases. There are very few new motherboards on the market that
use the full AT size. It is fairly common 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.
The Baby AT motherboard was, through 1997, the most common form factor on
the market. After three years and a heavy marketing push from Intel, the ATX form
factor is now finally overtaking the AT form factor and from here out will be the most
popular form factor for new systems. AT and Baby AT are not going anywhere,
however, because there are currently just so many baby AT cases, power supplies and
motherboards on the market.
A Baby AT motherboard is 8.5" wide and nominally 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. One problem with baby AT boards is that
many newer ones reduce cost by reducing the size of the board. While the width is
quite standard, many newer motherboards are only 11" or even 10" long.
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 that go between
the physical connectors mounted on the case, and pin "headers" located on the
motherboard.
The AT and Baby AT form factors put the processor socket(s)/slot(s) and
memory sockets at the front of the motherboard, and long expansion cards were
designed to extend over them. When this form factor was designed, over ten years ago,
this worked fine: processors and memory chips were small and put directly onto the
motherboard, and clearance wasn't an issue. However, now we have memory in
SIMM/DIMM sockets, not directly inserted onto the motherboard, and we have larger
processors that need big heat sinks and fans mounted on them. Since the processor is
still often in the same place, the result can be that the processor+heat sink+fan
combination often blocks as many as three of the expansion slots on the motherboard!
Most newer Baby AT style motherboards have moved the SIMM or DIMM sockets out of
the way, but the processor remains a problem. ATX was designed in part to solve this
issue.