Gary Urton and Carrie J. Brezine of Harvard University say they have found a three-knot pattern in some of the strings, called khipu, that they believe identifies them as coming from the city of Puruchuco, about seven miles north of modern Lima, Peru.
They used computers to analyze 21 khipu found at Puruchuco and divided them into three groups based on the knot patterns. Their findings are reported in Friday's issue of the journal Science.
One group seems to be for local use and the other two groups — each with the three-knot pattern — may have been used to report local activities to higher authority, or to receive messages from those authorities. Details of the information from the local khipu was coded onto the others intended for travel.