What were the Incan "Quipu" knotted strings used for?

"Quipu", or alternately spelled "khipu", are a collection of strings used as a device for recording information, such as for taxes, census information and other numeric values. Several South American cultures in the Andes Mountains, including the Incan Empire, utilized these for keeping information. Some of the earliest recognized specimens date from the first millennium AD, with wide-spread use up until the Spanish conquest of the Andes in the 16th century.

During Spanish rule in South America, the colonial authorities deemed quipus as an impediment to the spread of Christianity in the region and outlawed their use. However, use of quipus today has been noted in a couple communities. Their use is more for ceremonial activities rather than keeping records.

The information recorded on a quipu were decimal numbers, with the use a zero value (no knot in the position where a digit knot would be). A string would have a certain number of knots to represent a value, for knots in the "tens" place, a group of 4 knots would be the value of 40. However, some scholars who study the use of quipus have suggested additional purposes for these devices, such as use as a writing system.