Khipu, or Quipu, are a method of recording information in knots, which was used in South America during the Inka empire in the 15th century. Ancient Scripts explains:
The main content of quipus are numbers, which are expressed by knots on a section of rope. Unlike our “Arabic” numbers which uses ten different symbols for each digit (0 to 9), quipu makers tied multiple knots in a tight sequence represent a “digit”. Digits can range from no knots (empty space) representing zero, to nine knots representing nine. For example, seven knots in a sequence equals the digit 7.
Multiple sequences of knots represent “digits” that make up a number larger than ten.