The thirty-six divisions of the zodiac

The decans divide the zodiac into thirty-six ten-degree segments, three to each sign. They originate in Egyptian astronomy as a series of stars used to mark the hours of the night and the passage of the year, and were absorbed into Hellenistic astrology, where each decan received a planetary ruler and a set of images.

Two main schemes of rulership exist: the Chaldean order, which runs the seven planets in sequence through the decans, and the triplicity scheme, which assigns the decans of each element by that element's signs. Each decan also acquired pictorial images, the most famous being those of the Picatrix and the "faces" of medieval astrology.