The term quipu, from the Quechua word khipu, meaning "knot," refers to an accounting system used by Inca officials. A mnemonic system of cords of cotton or camelid fiber, a quipu consisted of a central or main cord positioned horizontally, from which hung other, secondary cords with knots tied in them to symbolize quantities. These cords varied in size, color, thickness, and type of braiding. Colonial documents describe a system of quipus to keep accounting records of people, livestock, and goods, and to gather information and send messages. The system continued to be used into the early years of Spanish colonial rule.
The quipus found to date display a great variety and complexity, the majority of them from the Inca era (1400 to 1532 ce). However, evidence from archaeological excavations indicates that the Incas did not invent the system. Quipus have been found from pre-Inca cultures such as the Caral (2,750 bce), the Wari (sixth to thirteenth centuries; Spanish, Huari), and the Chachapoyas (ninth to fifteenth centuries). The Incas perfected and made extensive use of this system, with specialists called quipukamayuq.
Ascher, Marcia, and Robert Ascher. Mathematics of the Incas: Code of the Quipu. Mineola, NY: Dover Publications, 1997.
Urton, Gary. The Social Life of Numbers: A Quechua Ontology of Numbers and Philosophy of Arithmetic. Austin: University of Texas Press, 1997.