The concept of decans originated with the ancient Chaldeans when they divided the 360 degree circle of the heavens into 36 equal parts of 10 degrees each, and each part was ruled by a planet or other heavenly body in the sky.
In modern times, however, the assignment of decans has changed considerably. Each sign is allocated a triplicity, such as air, water, earth or fire. Each sign is therefore subdivided into three equal parts of 10 degrees each, and these parts are referred to as decans, or decanates.
The decans and their rulers are assigned as follows as based on the concepts of modern Western astrology:
| Decan |
|Decan 20-30 degrees|