Decans

Each zodiac sign divides into 3 decans of 10° each, for 36 across the full zodiac. Their origin is Egyptian, much older than the 12-sign zodiac: decans were 36 small constellations that rose on the horizon roughly every ten days, used as a nighttime star clock. Only in the Hellenistic period were decans merged with the Greco-Babylonian 12-sign zodiac, gaining planetary rulers — and here two different assignment systems emerge, which disagree with each other in most cases.

Aries — 1st Decan (0°–10°)0°–10° of Aries — Mars doubled, the sign's purest energy.
Aries — 2nd Decan (10°–20°)10°–20° of Aries — the Sun adds expressiveness to Aries' drive.
Aries — 3rd Decan (20°–30°)20°–30° of Aries — the first point where the two systems disagree.
Taurus — 1st Decan (0°–10°)0°–10° of Taurus — Venus under Triplicity, Mercury under the Chaldean system.
Taurus — 2nd Decan (10°–20°)10°–20° of Taurus — Mercury under Triplicity, the Moon under the Chaldean system.
Taurus — 3rd Decan (20°–30°)20°–30° of Taurus — Saturn doubled, discipline over Taurus' base nature.
Gemini — 1st Decan (0°–10°)0°–10° of Gemini — Mercury under Triplicity, Jupiter under the Chaldean system.
Gemini — 2nd Decan (10°–20°)10°–20° of Gemini — Venus under Triplicity, Mars under the Chaldean system.
Gemini — 3rd Decan (20°–30°)20°–30° of Gemini — Saturn under Triplicity, the Sun under the Chaldean system.
Cancer — 1st Decan (0°–10°)0°–10° of Cancer — the Moon in its purest form, or Venus lending affection and beauty.
Cancer — 2nd Decan (10°–20°)10°–20° of Cancer — Mars tempers Cancer's sensitivity with intensity, or Mercury with mental agility.
Cancer — 3rd Decan (20°–30°)20°–30° of Cancer — Jupiter expands Cancer's care, or the Moon returns the sign to its emotional root.
Leo — 1st Decan (0°–10°)0°–10° of Leo — the Sun in its purest form, or Saturn imposing discipline on Leo's shine.
Leo — 2nd Decan (10°–20°)10°–20° of Leo — Jupiter doubled, expanding Leo's shine with generosity and optimism.
Leo — 3rd Decan (20°–30°)20°–30° of Leo — Mars doubled, adding direct action to Leo's shine.
Virgo — 1st Decan (0°–10°)0°–10° of Virgo — Mercury at its purest, or the Sun lending vitality to Virgo's precision.
Virgo — 2nd Decan (10°–20°)10°–20° of Virgo — Saturn's discipline or Venus's refinement layered onto Virgo's precision.
Virgo — 3rd Decan (20°–30°)20°–30° of Virgo — Venus's sensory touch, or Mercury at its most concentrated.
Libra — 1st Decan (0°–10°)0°–10° of Libra — Venus at its most concentrated, or the Moon's emotional sensitivity.
Libra — 2nd Decan (10°–20°)10°–20° of Libra — Saturn doubled, bringing structure and seriousness to Libra's pursuit of balance.
Libra — 3rd Decan (20°–30°)20°–30° of Libra — Mercury's social intelligence, or Jupiter's expansive generosity.
Scorpio — 1st Decan (0°–10°)0°–10° of Scorpio — Mars doubled, the sign's intensity at its most direct.
Scorpio — 2nd Decan (10°–20°)10°–20° of Scorpio — Jupiter or the Sun, two different readings of the same intensity.
Scorpio — 3rd Decan (20°–30°)20°–30° of Scorpio — the Moon or Venus, two different closing notes for the sign.
Sagittarius — 1st Decan (0°–10°)0°–10° of Sagittarius — Jupiter in its purest state, or a Mercurial layer of detail.
Sagittarius — 2nd Decan (10°–20°)10°–20° of Sagittarius — Mars accelerates the drive, the Moon deepens the emotion.
Sagittarius — 3rd Decan (20°–30°)20°–30° of Sagittarius — the Sun's shine or Saturn's discipline to close the sign.
Capricorn — 1st Decan (0°–10°)0°–10° of Capricorn — Saturn doubled, or a more expansive ambition, depending on the system.
Capricorn — 2nd Decan (10°–20°)10°–20° of Capricorn — ambition refined by Venus, or driven by Mars.
Capricorn — 3rd Decan (20°–30°)20°–30° of Capricorn — analytical ambition with Mercury, or vitality with the Sun.
Aquarius — 1st Decan (0°–10°)0°–10° of Aquarius — Saturn doubled, or an independence softened by Venus.
Aquarius — 2nd Decan (10°–20°)10°–20° of Aquarius — Mercury doubled, the Aquarian mind in its purest state.
Aquarius — 3rd Decan (20°–30°)20°–30° of Aquarius — independence with a touch of Venus, or of lunar sensitivity.
Pisces — 1st Decan (0°–10°)0°–10° of Pisces — Jupiter in Triplicity, Saturn in the Chaldean system.
Pisces — 2nd Decan (10°–20°)10°–20° of Pisces — the Moon in Triplicity, Jupiter in the Chaldean system.
Pisces — 3rd Decan (20°–30°)20°–30° of Pisces — Mars elegantly closes the 36-decan cycle.

Frequently asked questions

Why are there two different systems for decan rulership?: The Triplicity system assigns each decan the sign's own ruler, followed by the rulers of the other two signs sharing its element, in zodiacal order. The older Chaldean system assigns rulers in one continuous, unbroken cycle of the 7 classical planets across all 36 decans, without resetting at each sign. The two systems agree in only a few cases — there's no consensus on which is 'the' correct system, and both remain in use.
Does Vedic astrology have an equivalent concept?: Yes — called Drekkana, also dividing each sign into three 10° parts, but with its own rulership logic, distinct from the Western Triplicity and Chaldean systems.