I do not think this can be determined as it is so ancient and there are so many types, each likely independently invented.
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An Abacus has beads and strings.
pheonix wright
ABACUS is a name of a calculating apparatus - it is not an acronym.
If you are refering to Chinese abacus their earliest rudimentary design of abacus has 1/4 rod beads (quite similar to the later Japanese soroban abacus). Various other types of abacus design were also seen afterwards, but the advance type of 2/5 rod beads became standard and classic. Afterwards, the basic 1/5 rod beads particularly became the type of basic design. The Roman abacus may be not connected to the Chinese abacus.
ABACUS - Abundant Beads, Addition and Calculation Utility System