A quipu is a recording device generally associated with the Incas, who ruled Peru before the Spanish conquest. Quipu consisted of a number of color-coded cords; knots of various kinds were tied on these cords to represent a variety of information. An important use of quipu was to record numbers for use in trade, keeping accounts, and calendars, but the knotted strings might also have served as mnemonics for important historical events, astronomical data and mythology (the Incas had no written language). Some evidence supports the idea that the knots and cords followed in a decimal system. Similar devices were used by several other Indian tribes and also described in Chinese and Persian documents from the 5th and 6th centuries BC, and are still used by shepherds in the Andes for keeping accounts of their herds.