For each of the signs of the zodiac, there is the sign ruler, the ruler of the decanate (10 degree chunk) as well as the rulers of each duad, which is a 2 degree, 30 minute section of the sign.
Decanate rulers are the rulers of the three signs with the same element. So the decanates of Aries, a fire sign ruled by Mars, are ruled by the rulers of the three fire signs: The first decanate of Aries is the Aries decanate, ruled by Mars. Then comes the Leo decanate, ruled by the Sun; & then the Sagittarius decanate, ruled by Jupiter. Astrological short-hand says the first decanate of Aries is the Aries decanate, the second is the Leo decanate, the third is the Sagittarius decanate. Moving along to the next fire sign, the first decanate of Leo, a sign ruled by the Sun, is ruled by the Sun, the second decanate of Leo (Sagittarius) is ruled by Jupiter, the third decanate (Aries) is ruled by Mars. Same planets, in the same order, ruling the same signs. Memorize these simple rules & you will know what planet rules what decanate.
Divide a sign of 30 degrees by 12 & you get two & a half degrees, or a Duad. Each of these are ruled by a sign of the zodiac, starting with the same sign as that of the overall sign itself.
So the first duad of Aries is Aries. The last duad of Aries is Pisces. Each sign is self-contained, the duad series does not go on from one 30 degree sign to the next. So in Taurus, the first duad is Taurus, the last is Aries, etc. (The duads are not like planetary hours.)
It's a bit tricky working out which duad applies to which degree, but it can be done. Here's how.
As each decanate contains a third of the "duad zodiac", the first decanate of Aries contains the duad signs of Aries through Cancer. By extension, the second decanate of Virgo (10-20), the Saturn/Capricorn decanate, contains the signs Capricorn, Aquarius, Pisces & Aries. So 14 Virgo is in the Capricorn decanate & the (second) Aquarius duad. All you need do is think in terms of quarters: 2.5 / 5 / 7.5 / 10. In the second decanate, duads fall at 12.5 / 15 / 17.5 / 20. In the third, 22.5 / 25 / 27.5, etc.
Working out details like this is good mental exercise.
You use these to extract more information about planets in signs, the ascendant degree, cuspal degrees. Determine the decanates & duads of all the planets & cusps in a chart, reduce to elements, and you may see some interesting patterns emerge. In terms of individual planets, you say Mars is in the second decanate of Scorpio? That's the Pisces decanate, ruled by Jupiter. So you would think, Mars, encouraged by Jupiter, is going to act a lot more in the second decanate than the first, and is not going to be quite as intensely focused, since Pisces difuses everything. If that Mars is at, say, 16 Scorpio, it will be in the (wait, let me think: A third down from Scorpio is Pisces, which would be the duads of Pisces, Aries, Taurus AHA!) Taurus duad which makes it a bit willfully stubborn, a Taurus trait.
You can apply these observations to the planet itself, the house it is in, the house it rules, and any aspects. Gives you more to work with. The authors delineate each decanate, and every duad, all 144 of them.
AFA, 116 pages.
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