A ball of string tied into countless knots could very well be seen as a source of frustration. But for the ancient Inca civilization, carefully tied knots formed the basis of a method of recordkeeping known as khipus. Now researchers report that the ledger system is more complex than previously believed and includes a way of communicating information to higher-ups in the well-categorized Incan chain of command between workers and administrators with higher rank.

Hundreds of khipus, each consisting of a single strand of wool from which hundreds to thousands of other knotted strings hang, have been discovered to date. Gary Upton and Carrie J. Brezine of Harvard University designed a computer program to analyze the patterns in 21 khipus recovered from a site in Puruchuco, an Inca administrative center on the coast of Peru near modern day Lima. They discovered that certain patterns within the strings of varying colors and lengths appear to contain numerical data that represent summations. What is more, the information is arranged among the khipus in a ranked pattern with three levels of authority. Information is passed between them by including the sum from a khipu at one level on a khipu representing a higher level.

"This work gives us some sense of how this complex information was compiled, manipulated, shared and archived in the Inca hierarchy," Urton says. "Instructions of higher-level officials for lower-level ones would have moved via khipu, from the top of the hierarchy down." Commands for the society's large labor force could have been shared in this way, the authors suggest in a paper published today in the journal Science.