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AT and Baby AT
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. These will need an upgrade path and I believe that at least some companies will make motherboards for the newer technology in AT form factor for some time, to fill this upgrade market demand.
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. There are three rows of mounting holes in the board; the first runs along the back of the board where the bus slots and keyboard connector are; the second runs through the middle of the board; and the third runs along the front of the board near where the drives are mounted. 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. This can lead to mounting problems, because the third row of holes on the motherboard won't line up with the row on the case. (Some reduce or skip the third row entirely). Fortunately, it is almost always possible to solidly mount the motherboard using only the first two rows of holes, and then using stubbed spacers for the third row. See the Motherboard Physical Installation Procedure for more perspective on these issues.
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.
Next: ATX and Mini ATX