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AT Form Factor
The "new" IBM PC/AT didn't look all that much different on the outside from the PC/XT units it replaced; but on the inside it was a different story. The AT case was functionally similar the older PC/XT style, but was changed slightly so that it represents a different form factor. The power supply was larger than in the PC/XT and the positioning and size of the motherboard and power supply different. Therefore, the PC/XT and AT formats were not compatible.
The AT form factor was very popular in the late 1980s, and was the basis of many "clone" manufacturers' units (for compatibility with IBM.) The AT system was also the first to formally introduce different desktop and tower configurations, as described on this page discussing the AT form factor power supply. The desktop configuration was very similar to that of the PC/XT, with the familiar red toggle switch in the rear of the machine, on the right-hand side. The tower configuration saw the introduction of the now familiar "remote" power switch, controlled by a button on the front of the case. Due to its convenience, this was very popular and became the standard for most later designs, both tower and desktop.
Next: Baby AT Form Factor