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The AT motherboards are the first modern form factor to be widely used. AT stands for Advanced Technology & it was released by IBM in 1984. The PC and XT form factors that preceded it were quite uncommon. There are a few computers that still use AT and its variant Baby AT, but current standards are in favor of ATX.
AT was not a huge technological advancement from older form factors. The biggest advancement was the "tower" design and a power switch in the front. The AT power supply provided 192 watts, thrice more than older form factors.
The AT motherboard is 12 inches (305 mm) and it won't fit in "mini-tower cases" or "mini desktop". One of the major problems with the width of this board it takes up space behind the drive bays making installation of new drives difficult. The power connectors for AT motherboards are almost identical with 6-pin cords. The two connectors are not physically distinguishable, leading to damage of the motherboard when they were connected incorrectly.
