Decanates
The twelve signs have been traditionally subdivided into decanates or decans, each of ten degrees. Tropical astrology copied this practice from ancient sidereal astrology, which accorded rulership of one third of each constellation to the most prominent star in it. There are two principal conventions:
1. The most common in use today involves the attribution of a secondary sign influence from the same sign triplicity (or element) to each decanate of each sign.
By this system, the first decanate is considered the pure or physical form of the sign; and planets posited therein should be interpreted with reference to their aspects to Mars in the nativity.
The second decanate takes a secondary influence from the next sign of the same element after it in the zodiac, and is considered the mental form of the sign, any planet posited therein being interpreted with reference to its aspect to Mercury in the figure.
The third decanate takes a secondary influence from the previous sign of the same element, and is considered the spiritual form of the sign, any planet posited therein being interpreted with reference to its aspects to Jupiter and Saturn in the figure, pinpointing how the spiritual will manifest (Jupiter) and where it will be tested (Saturn).
According to Bernice Grebner, the decanate influence shows the purpose of the sign placement for the individual, while the dwad influence shows the conditions influencing the individual, or the means of dealing with problems presented by the placement.
