Computers come in different form factors. ATX is the most common. AT used to be the standard but is now obsolete. NLX and LPX are two others. These forms describe the shape and size of the motherboards, as well as the layout of the components on the board. The form factor will also determine the type of case you must buy, as the case is laid out differently and uses a different type of power supply.

AT Form Factor

Within the AT form, we have regular AT and Baby AT. They basically differ in size. An AT board is about 12″ wide which means it can’t fit in many of today’s cases. AT boards generally are the older boards, 386 or earlier. Working inside the case was a lot more trouble with these because the size of the motherboard overlapped drive bays and such.

Baby AT is the form used by many 486 and Pentium boards.. Many Socket 7 motherboards and a few Pentium II boards used this form factor. A Baby AT board is roughly 8.5″ wide and 13″ long. The size varies a little from board to board. This reduced size makes it easier to work inside the case simply because there is more room. There are three rows of mounting holes to hold the board in the case.

AT form boards share common traits. They all have serial and parallel ports attached to the case in an expansion slot and connected to the board through cables. They also have a single keyboard connector soldered onto the board at the back of the board. The processor is still at the front of the board and can sometimes get in the way of expansion cards. The SIMM slots are in different places, although they are almost always at the top of the board.

There are some annoyances with the AT design. One is due to the layout. Since all ports are attached to the case and then connected to the motherboard via a cable, the board must have connectors for all of these: COM 1, COM 2, printer port, USB, PS/2 mouse, etc. Often these connectors are directly next to the IDE channel connectors and floppy drive connector. This leads to a severe cramping problem and makes working inside the computer more difficult. Secondly, the AT design is not conducive to efficient cooling of the system. Air is not blown over the areas that need it, namely the CPU. Also, the air flow draws in dust. Over time, the AT power supply will get dusty and the inside of the system will be coated with a layer of dust. For this reason, it is recommended you regularly remove the case and blow off the interior of the case.

You can see a diagram of an AT motherboard here: AT Socket 7 Motherboard. Here is some additional information on PC form factors: A+ Guide 2: Cases, Motherboards, and Processors.