Rulers made of Ivory were in use by the Indus Valley Civilization period prior to 1500 BC. Excavations at Lothal (2400 BC) have yielded one such ruler calibrated to about 1⁄16 in (1.6 mm). Ian Whitelaw holds that the Mohenjo-Daro ruler is divided into units corresponding to 1.32 in (33.5 mm) and these are marked out in decimal subdivisions with amazing accuracy, to within 0.005 in (0.13 mm). Ancient bricks found throughout the region have dimensions that correspond to these units.
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