How can knots keep a record of numbers?

A number does not have to be written in ink. In the Andes, skilled record keepers used knotted cords called khipus, also spelled quipus. Inka officials used them to keep records. Many surviving examples contain decimal numbers.

In a decimal khipu, position matters. Groups of single knots can show hundreds, tens and larger places. At the ones level, different knot forms are used: a long knot has turns for values two through nine, while a figure-eight knot can show one. The invented cord in the picture has two hundreds, one ten and five ones. Together these make 215.

An unknotted stretch at a known level can mean zero. That makes a number such as 304 possible: three hundreds, no tens and four ones. But identifying a number is not the same as knowing what was counted. Cord arrangements, colors and historical context matter. Researchers are still investigating meanings in khipus; the picture is a teaching example, not a translation of an ancient record.