(from wikipedia): The Curta is a small mechanical calculator developed by Curt Herzstark. The Curta's design is a descendant of Gottfried Leibniz's Stepped Reckoner and Charles Thomas' Arithmometer, accumulating values on cogs, which are added or complemented by a stepped drum mechanism. It has an extremely compact design: a small cylinder that fits in the palm of the hand. Curtas were considered the best portable calculators available until they were displaced by electronic calculators in the 1970s. See also: http://curta.org/wiki Check this video: https://youtu.be/loI1Kwed8Pk