The original purpose of such a calendar, with no obvious relation to any astronomical or geophysical cycle, is not securely known, but there are several theories. One theory is that the calendar came from mathematical operations based on the numbers thirteen and twenty, which were important numbers to the Maya, (Thompson 1950: Maya Hieroglyphic Writing:Introduction). The number twenty was the basis of the Maya counting system, taken from the total number of human digits. (See Maya numerals). Thirteen symbolized the number of levels in the Upperworld where the gods lived, and is also cited by modern daykeepers as the number of "joints" in the human body (ankles, knees, hips, shoulders, elbows, wrists, and neck). The numbers multiplied together equal 260.