Source : Free On-Line Dictionary of Computing

IBM PC AT
     
         ("Advanced Technology") A version of the {IBM PC},
        released in Aug 1984 with an {Intel 80286} processor, a 16-bit
        {bus}, a medium-speed {hard disk} and a 1.2 {megabyte}
        {floppy} {disk drive}.  It had a larger case than the PC,
        which allowed it to accept "{tall cards}".
     
        The AT keyboard corrected the PC's non-standard placement of
        the {return} and left shift keys but shortened the {backspace}
        key, making it harder to reach.
     
        (1995-03-01)