The architecture developed by IBM and opened up for use by other manufacturers. Renamed ISA, it became the de facto architecture for personal computers for many years. It was introduced by IBM in the 8-bit IBM PC/XT and expanded when the 16-bit PC/AT was introduced. It allowed add-in cards to be plugged into standard expansion slots. A 16-bit ISA slot comprised two separate 8-bit slots. See also EISA, local bus architecture, MCA.