HP-25

The HP-25 started on the drawing board as a scientific calculator with a much larger set of functions than the HP-21. By the time it was done, it was that and it was programmable with 49 lines of memory, making it the least expensive and smallest of HP's programmable calculators. Like the HP-65, the HP-25 caused disbelief in many. It seemed much too small to do so much. Unlike previous programmables, the HP-25 had fully merged key steps... (All multi-keystroke entries like f COS and STO * 2 took a single step.) Engineering display mode was also a first on this calculator.