The Addiator was a mechanical add/subtract calculator, made by Addiator Gesellschaft, Berlin. Variants of it were sold from 1920 until 1982. It was composed of sheet-metal sliders inside a metal envelope, manipulated by a stylus, with an innovative carry mechanism, doing subtract ten, carry one with a simple stylus movement.
Only made obsolete by the electronic variety, it was simple and cheap. It also handled non-decimal measurements, like feet and inches, or pre-decimalization pounds, shillings, and pence. Addition and subtraction required different 'screens', handled by turning the instrument over, or flipping a front panel, or, later, by extended sliders and an extra lower panel.
More expensive versions had a built-in slide rule on the back.
This type of calculator was introduced by the Frenchman Troncet in 1889. The Addiator was one of the most popular calculators of this sort, and the name is often used to refer to the type generally.