Old Calculator is a term used to describe two different tools used for calculations in the past: an ABACUS and a slide rule. An ABACUS is an ancient calculating tool consisting of a frame with beads that can be moved along rods to perform mathematical operations. A slide rule is a mechanical device used to perform mathematical calculations, consisting of two or more logarithmic scales that slide against each other. Both of these tools were used before the invention of the modern calculator.