At the heart of the Inca Empire was the khipu, a system of knotted cords used to record quantities, much like a spreadsheet. Units, tens, hundreds and even tens of thousands could be tracked through carefully placed knots. While specialists can read the numbers, much about their meaning remains unknown. What is clear, however, is that khipus were essential to managing the empire’s political and economic life. Watch Incas: The Rise and Fall, Sundays at 8pm, on National Geographic UK | National Ge