String of Stones
From Attu Project
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The String of Stones is a children's toy, originally invented around 180 TT by Wesian inventor Joric Iri-Praica for the purposes of trigonometric calculation. It consists of a long string affixed to two wooden boards. Along the string are a series of detachable stones. Children typically play with the stones by moving them from one board to the other, and vice versa.