Quipu, South American, c 15th century.
© Science Museum / Science & Society Picture Library
Replica made in 1974 from an original held at the Museum of Natural History, New York. Quipus were used by the Incas during the 15th and early 16th centuries as records of all types of accounts. It was discovered in the early 20th century that the knots represent numbers on a base 10 system; the long knots being units and the short ones tens, hundreds or thousands depending on their position.