The IBM PC AT is the successor of the PC and the XT. IBM added a lot of new features including switching to the Intel 80286, which allowed for among many other things, 16 bit expansion slots.
It also used then-new MS DOS 3.0 which supported 1.2 MB floppies, 20 MB disks, and allowed file sharing. The keyboard also came with cursor keys and a key that could lock it. Two models were launched: the PC-AT model 1 (256 KB RAM, two floppy disk units and a color screen) and the PC-AT Model 2 (512 KB RAM, one floppy disk unit, one hard disk and a color screen).
This computer was revolutionary, but it was the last time IBM imposed a standard onthe PC clone industry. The next year, the first PC based on a 80386 was made by Compaq and IBM failed to capitalize on the PS/2 standard in 1986.