In order to give fuller interpretation to the zodiac signs, ancient astrologers subdivided each sign into periods of approximately ten days. These divisions are known as the "decans" or "decantes" and are somewhat arbitrary in order to allow for the five (and sometimes six) extra days in the year beyond the 360 days required for the thirty-six decans. 
Astrologers believe that each of the decans has its own individual character.
In modern times, however, the assignment of decans has changed considerably. Each sign is allocated a triplicity, consisting of three of the four classical elements air, water, earth or fire, and is therefore subdivided into three equal parts of 10 degrees each; 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:
|Sign||0-10 degrees||10-20 degrees||20-30 degrees|
The decans and their rulers are assigned as follows as based on the concepts of ancient Western astrology :
|Sign||Domicile ruler||Exaltation ruler||0-10 degrees||10-20 degrees||20-30 degrees|
- According to novareinna.com - http://www.novareinna.com/constellation/scorpiodecans.html Archived 2010-01-03 at the Wayback Machine