Comptometer

A comptometer is a special machine that helps people add and subtract numbers quickly and easily before calculators existed. It has buttons that you can press down to add, and when you release the buttons, it adds the number to your total display. You can also subtract numbers by pressing special buttons, and the machine will subtract that number from your total display. It's kind of like a very simple calculator that you don't need to plug in or use batteries, and it's small enough to fit on a desk. People used comptometers a lot in the past, but now most people use electronic calculators instead.