While they were still used, my mother was a Comptometer operator. https://www.jaapsch.net/mechcalc/comptometer.htm https://www.jaapsch.net/mechcalc/comptomete

While they were still used, my mother was a Comptometer operator. https://www.jaapsch.net/mechcalc/comptometer.htm https://www.jaapsch.net/mechcalc/comptometer.htm To use one required, for operations more complex than addition (e.g. division), an understanding of arithmetic algorithms - similar to those used in a CPU, except in decimal.