The three groups or families of computers described in this part are each built around a single ISP and PMS structure. The IBM 701-7094 II sequence (Sec. 1) shows the evolution of a series. The reader can trace a number of incremental changes, or features, such as the addition of index registers, indirect addressing, I/O processors, and larger random-access memories. The SDS 900-9000 series and the IBM System/360 are both families in which successor models are within a planned framework; evolution occurs mainly in the implementations, not in the ISP.