In AT motherboards, AT stands for “Advanced Technology”.  AT Motherboard was a classic/traditional motherboard that was introduced by IBM in the 1980s. The motherboard size was  13.8 x 12 inches and shaped approximately double than the other functional motherboards. Due to its large size, it was not used in small size desktop computers. The large size is because of the fact that it used 386 processors and installing, servicing, and upgrading it was a big challenging task, due to its complicated layout.