Napier's bones

John Napier described his rods in 1617, the year he died, as a way to turn multiplication into addition. Each rod carries a multiplication table split into squares, and every square is cut by a diagonal with the tens digit of the product above it and the units digit below. Lay rods for each digit of a number side by side, read across the row for the multiplier, and add the digits that fall between neighbouring diagonals. No mechanism is involved, yet the trick is a real algorithm made physical, and the best sets were carved in ivory, hence the name. Their influence was direct: Wilhelm Schickard built a set of these rods into his calculating machine within a few years.