If you needed a desktop calculator 85 years ago, you couldn’t go wrong with the one described in the October 1938 issue of the British magazine Practical Mechanics.

It was a literal desktop calculator, since you affixed it to your desk. It consisted of three logarithmic scales, and could perform multiplication and division, and even square roots. To multiply, you placed your straightedge on the numbers on the outer scales. At the point where it crossed the middle scale, that was the product.

You could use any ruler, but the magazine recommended as the best option a strip of celluloid. You would carefully score a line down the middle, fill it with ink, and then polish away the excess ink.

With practice, the calculator was accurate out to three figures.