An abacus is a simple mechanical tool used for counting and basic mathematics. It consists of a frame that features several rods or strings, each rod containing several beads that may be slid to one side or the other.

Each rod stands for a decimal place. Most abaci have seven to ten rods. To simplify computation, the fifth and/or sixth beads are often marked or colored to distinguish them from the others.

Abaci appeared thousands of years ago in China and spread to Europe through India and the Arab world.