Complex sentence: The IBM 709 had a 38 bit accumulator, a 36 bit multiplier quotient register, and three 15 bit index registers whose contents were subtracted from the base address instead of being added to it.
Split into simpler sentences: The IBM 709 had a 38 bit accumulator, a 36 bit multiplier quotient register, and three 15 bit decrement registers. The decrement registers were a kind of index register whose contents were subtracted from the base address instead of being added to it.