The
form factor used by most
PC motherboards prior to 1998. The original motherboard for the
PC-AT measured 12"x13". Baby AT motherboards are a little smaller, 8.5"x13" usually. The Baby AT is being replaced by the new
ATX form factor.
Also see
From AT to BTX: Motherboard Form Factors in Webopedia's "Did
You Know...?" section.