Khipus (or quipu, in Spanish) were used by the Incas of South America. A khipu is a collection of knotted strings that record certain information. The approximately 600 surviving khipus use an arrangement of knotted strings hanging from horizontal cords. But these knots are nothing like those made by other cultures: They include long knots with four turns, single knots, figure-eight knots, and a whole host of other knot types. Historians believe these strings and knots represent numbers once used for accounting, inventory, and population census purposes.

There are also researchers who believe the khipus may contain certain messages in some sort of code—a kind of language used by the Incas—based on the strings, knots, and even a khipu string’s type (usually alpaca wool or cotton) and color. But it may turn out that historians will never know the real story behind the khipus. When the Spanish conquered the Inca Empire starting in 1532, they destroyed most of the strings, believing they might be idolatrous items containing accounts of Incan history and religion.