The Free On-line Dictionary of Computing (30 December 2018):

IBM PC AT
AT
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)