Roman abacus

A reconstruction of a Roman hand abacus, made by the RGZ Museum in Mainz, 1977. The original is bronze and is held by the Bibliothèque nationale de France, in Paris. This example is missing many counter beads.
Velser's reconstruction of Roman abacus (ca. 1600)

The Roman hand abacus was one of the earliest known portable calculating devices, used in the Roman world. It was a base-10 form of the abacus, with movable beads held in slots rather than loose counters. The device belonged to a wider Mediterranean tradition of counter-based calculation that included Greek reckoning boards, although its exact developmental lineage is not documented. Its beads represented values in decimal place-value columns, allowing calculations to be carried out on the device, while Roman numerals were used mainly to record results. The device also included positions for fractional values used in Roman measures and Roman currency, especially the uncia, or one-twelfth of a unit.

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