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khipu.jpgelaborate structures of coloured, knotted strings that were used to physically represent census & tribute data, synthesized, manipulated & transferred between different accounting levels in the Inka Empire administrative system (Peru) about 500 years ago. recent research indicates that khipu encode numbers as knots in strings hanging from a cord: the closer a knot is to the cord, the higher its value, just as the number 1 can denote 1, 10, or 100 depending on its position. numerical values depend on the knot's shape: single overhand knots encode tens, hundreds or thousands. single knots represent ones, & long knots encode the numbers 2 to 9. [sciencemag.org & nature.com]