The abacus — a counting board with positional columns — is the first documented physical calculating instrument. In Mesopotamia, counting boards with grooves or clay pebbles are attested from the 3rd millennium BC in Sumerian and Akkadian accounting contexts; in China, the suanpan with beads on rods is documented with certainty from c.200 BC (Han period) and probably earlier. Both developments are independent: there is no evidence of transmission between traditions. The abacus remains in commercial use in East Asia until the 20th century and is still an active teaching tool. It is the first documented piece of the instrumental chain that leads to the mechanical calculator and the computer.