In addition to verbal messages conveyed by runners, information was relayed across the roads and bridges of the Incan empire through the passing of items known as khipus (sometimes spelled "quipus").
These communication devices consisted of a main cord (the primary cord) from which a series of knotted strings of varying length and color were suspended (pendant cords). The strings were woven from cotton or the wool from llamas or alpacas. It's believed that the number of knots -- as well as knot types and their position on each pendant cord -- was used for record keeping according to a decimal system. The cords were likely used to keep stock of various commodities stored in qolqas, or warehouses, that were located across the empire.
To this day, unlocking the messages contained in the khipus has been impossible, but researchers at Harvard University have begun the Khipu Database Project. Started in 2002, the project is attempting to centralize all known information about khipus and feed it into a database that will compare and contrast various patterns of khipu construction. The hope is to develop a Rosetta stone of sorts that will untangle the ancient messages hidden in the strings.