Left to right: (1) desktop-size Thales pinwheel calculator, 1910s, four operations, 13 digits with bell on overflow; (2) Curta Type I, pocket size, 1950s, stepped drum, 11 digits; (3) Curta Type II, sligthly larger,15 digits; (4) Alpina, portable, 1960s, pinwheel, 13 digits.