a pinwheel calculator was a class of mechanical calculator popular in the 19th and 20th century using, for its calculating engine, a set of wheels that had an adjustable number of teeth. these wheels, also called pinwheels, could be set by using a side lever which could expose anywhere from 0 to 9 teeth, and therefore when coupled to a counter they could, at each rotation, add a number from 0 to 9 to the result. by linking these wheels with carry mechanisms a new kind of calculator engine was invented. turn the wheels one way and one performs an addition, the other way a subtraction. as part of a redesign of the arithmometer, they reduced by an order of magnitude the cost and the size of a mechanical calculators on which one could easily do the four basic operations (add, subtract, multiply and divide).