Abacus

The abacus, a mechanical aid to performing arithmetic, dates back many centuries and is still used in modern times.

The Roman hand abacus is estimated to have been created some time between 300 B.C. and 500 A.D. An advancement of the hand abacus was the implementation of permanently attached markers, which are adjusted in position to indicate value. The suan-pan, or Chinese abacus, is known to have existed already by around 1200 A.D.

Usage of an abacus relies on a concept of "states" and place values; that is--whether or not beads are in the "inclusive" or "not-inclusive" positions. To count items on an abacus, a number of beads are shifted over to the represented position that indicates a counted value, and any that are not moved are not counted. A typical modern-day abacus has slidable markers are placed on columns of shafts (typically made from wood or metal) representing powers of ten (0.0001, 0.001, 0.01, 0.1, 1, 10, 100 etc), with the top row representing values of "fives" and the bottom representing values of "ones". These markers are permanently attached to the device.