The Inca, a civilization that dominated the western part of South America for a few centuries in the early Middle Ages, did not have a written language. The closest thing they had was the quipu.

A quipu had a main cord (a string or wooden bar), off which dangled a number of cotton or wool strings. Each string had a number of knots, in different locations up and down the strings, which were of different colors and were often twisted in different patterns–all on a decimal system. Such patterns indicated various things on various levels: basic things like dates, statistics, and other number-based information; and more abstract things like ideas and concepts. Different colors (and historians have identified as many as 52) signified different things. Varying as well were the total number of knots tied on a string and the position of each knot on that string. The result was a large number of possible combinations for conveying information.

One theory put forward in recent years is that a quipu could be used as a mnemonic device, to help people remember the elements of traditional legends and stories.

The Inca used quipus for keeping records and for sending messages–from one individual to another and, more dramatically, from one part of the empire to another. The Inca road system stretched more than 25,000 miles across the far-flung geography of the empire. A system of relay runners could transmit one or a handful of quipus from one set of hands to another, covering up to 150 miles in a day.

The Quechua word for the English word "knot" gave rise to the Spanish word quipu. A spelling closer to the original is khipu.

As part of the conquest of the Inca, the invading Spanish destroyed thousands of quipus. The Spanish did write about them, though, which is how people today know about the quipu. As well, some quipus survived. An estimated 600 can be found in museums around the world or tucked in communities in lands that were once ruled by the Inca or their predecessors.

The Inca did not invent the quipu. Historians have found a handful that predate the Inca dominance of South America. One of the oldest quipus yet found dates to the Caral civilization, which arose about 4,000 years ago.