The Khipu-Based Numeration System
  This paper presents a survey of the khipu-based enumeration system of the Incas. In this system, different knots are tied together and they are attached to a primary cord. The numerical basis of the khipu system is well understood but more recent theories suggest that the khipu also encoded narrative information and this may have been facilitated by binary mapping. 
Sneha Pilgaonkar
math.HO