Any house system can use decanates, but they are the hallmark of the Hellenistic whole house system. Decans, which is how one refers to an individual member, refers to the subdivision of a house into ten degree “triplets”, while decanates refers to the entire group. Decans always are in the same order for each sign no matter what sign starts the chart.
To make this easy to visualize, I am using Senator Kyrsten Sinema of Arizona’s natal chart with decans in the natural house system, also called the zero Aries start system. Looking at the chart below, the decans are the second house from the outside, and are shown with the planets in chartreuse on a black background.
Mapping lords to rulers
The first house is Aries and its thirty degrees are broken into three triplets of ten degrees each. We assign each triplet the planetary lord of each sign within the same quadrature. For Aries, its ruler is Mars; rulers and lords are synonymous. The next decan belongs to the next sign in the fire quadrature: Leo and its ruler is the Sun, while the final decan of the fire quadrature is Sagittarius, whose ruler is Jupiter. They are always three signs comprising a quadrature and there are four quadratures.
The second house is Taurus. The first decan is Venus, the lord of Taurus, followed by Mercury, the ruler for Virgo and Saturn the ruler of Capricorn, thus all the lords for the earth quadrature are shown.
The third house is Gemini. Its lord is Mercury, the second decan is Venus for Libra, and the third is Saturn, the lord for Aquarius because this is a traditional setup and the Herschel’s did not discover Uranus until the eighteenth century long after they created this system. Many modern astrologers change this attribution to Uranus instead of Saturn, but the traditional association is Saturn rules Aquarius.
The fourth house is Cancer and the first triplet is the Moon who rules it. The second triplet is Mars for Scorpio. Like Saturn ruling Aquarius, Pluto is not shown, unless you choose to make that change. The third and final triplet is Jupiter for Pisces, because they did not discover Neptune until the nineteenth century. With Cancer, we have completed the first set of quadratures for the zodiac. We will repeat this three more times.
The Middle Four
The fifth house is Leo. On Senator Sinema’s chart that shows up with the number 9 on the outmost circle because had we used a typical natal system, the ascendant would have been the starting house, and Leo would be in the ninth house. All the houses have that dual numbering system on the chart above. The ruler for Leo is the Sun — remember this is the signs rulership not its exaltation, which for the Sun is Mars. That would not work because the luminaries only have one planet in exaltation, and the other ten have two thus it would leave us with a missing decan. The second decan is Jupiter for Sagittarius and the third is Mars. Notice that while Aries started with Mars and ended with Sagittarius, Leo starts with its own ruler with the second decan always being the next planet in the quadrature, which in this case is Sagittarius and not Aries.
The sixth house is Virgo. Like Gemini, its ruler is Mercury, but the following sign is Capricorn thus the next decan is Saturn. The final decan is Venus for Taurus. This is the second sign in the earth quadrature.
The seventh house is Libra, always opposite Aries. Its ruler is Venus, the next decan is Saturn for Aquarius and the third decan is Virgo for Gemini. This is part of the air quadrature.
The eighth house is Scorpio. On this chart its outer number is eleven, but following the Aries start or Natural House method, I show it as the eighth house. Scorpio’s ruler, like Aries, is Mars, the next decan is Jupiter for Pisces, and the third decan is the Moon for Cancer.
The Final Four
The ninth house is Sagittarius. Its ruler is Jupiter, like Pisces. The second decan is Mars for Aries and the third decan is Leo, as we work counter clockwise to fill in the lords. With Sagittarius, we have finished the fire quadrature.
The tenth house is Capricorn. Its ruler is Saturn, the second decan is Venus for Taurus and the third decan is Mercury for Virgo. We have now finished the earth quadrature.
The eleventh house is Aquarius. Its ruler is also Saturn, and not Uranus. The second decan is Mercury for Gemini and the third decan is Venus for Libra. We have now finished the air quadrature.
The twelfth and last house in the zodiac is Pisces. Its ruler is Jupiter, the second decan is the Moon for Cancer and the third decan is Mars, and not Pluto, for Scorpio. We have now finished the water quadrature and completed all the decans for the signs too.
Sinema’s ascendant rising chart
Below is Senator Sinema’s traditional natal chart.