The Inka “wrote” by tying knots in strings. Bundles of knotted strings are called khipus; these typically recorded decimal numbers in the thousands. Inka number-words up to ten to the eighteenth power, a quintillion, were documented in 1607 by a Jesuit missionary. Inka numbers are widely considered to be positional with place value and a zero (a blank section of string), but this interpretation may reflect familiarity with Western numbers. Inka numbers were more likely additive, organized like the Roman numerals. No chronicler described Inka calculations beyond noting that kernels of maize were used as counters, and these were moved with speed and skill. Three devices have been proposed for the yupana, the Inka calculating device. The first is a small, portable artifact with multiple levels and partitions, though it is likely from Ecuador and had uses other than numbers. The second is a checkerboard drawn in 1615; it has been subject to multiple mathematical analyses, with no agreed-upon interpretation. The third is the Inka storehouses whose stone floors are pitted with regularly spaced holes, though their size, depth, and spacing would make them difficult to use with counters.