Decans

via Tarot.com :

"If you understand Triplicity in Astrology, then you can grasp what a Decan is. Triplicity is a fancy term to explain the different elements in our zodiac. Triplicity is a group of three signs (triple) that belong to the same element. There are three Fire signs (Aries, Leo, and Sagittarius), three Earth signs (Taurus, Virgo, and Capricorn), three Air signs (Gemini, Libra, and Aquarius), and three Water signs (Cancer, Scorpio, and Pisces).

It's all about the degrees

Now, it's important to remember that there are thirty degrees in every sign -- no matter what sign we're talking about. [...] Decans are a nifty way of breaking down the qualities of each individual sign by separating each one into thirds of ten degrees each. Every sign will have three Decans and each Decan has a planetary ruler which becomes the co ruler of that particular sign. By knowing which Decan a planet or other point in your horoscope is in, you can learn more information about its story and how it may affect your psyche.

Every Decan must belong to the same Triplicity (element) as the sign itself. Decans are arranged in the exact order they would appear in the natural zodiac wheel. If we use Aries as an example, we can see that the first Decan of any planet in Aries will apply for the first ten degrees. It will be an Aries-Aries Decan which is pure, unaltered Aries energy, ruled only by Mars. The first Decan of any sign will correspond to the exact sign, giving it an extra pure expression of that particular energy.

If you have a planet between ten and twenty degrees of Aries however, there is a slight change. Now we're in the Aries-Leo Decan ... which gives a different flavor. This planet will have the Sun as a co-ruler, adding more dimensions to interpretation. If you've got a planet between twenty and thirty degrees of Aries, then it falls in the Aries-Sagittarius Decan ... which has a Mars-Jupiter quality to it. You can quickly see how someone born with a 5 degree Aries Sun, a 15 degree Aries Sun and a 25 degree Aries Sun will all have varying expressions of their solar energy. Fascinating!

[...]

Aries: Fire, Ruled by Mars

First Decan: 0-10 degrees, ruled by Mars

Second Decan: 10-20 degrees, ruled by Sun

Third Decan: 20-30 degrees, ruled by Jupiter

Taurus: Earth, Ruled by Venus

First Decan: 0-10 degrees, ruled by Venus

Second Decan: 10-20 degrees, ruled by Mercury

Third Decan: 20-30 degrees, ruled by Saturn

Gemini: Air, Ruled by Mercury

First Decan:0-10 degrees, ruled by Mercury

Second Decan: 10-20 degrees, ruled by Venus

Third Decan: 20-30 degrees, ruled by Uranus

Cancer: Water, Ruled by Moon

First Decan: 0-10 degrees, ruled by Moon

Second Decan: 10-20 degrees, ruled by Pluto

Third Decan: 20-30 degrees, ruled by Neptune

Leo: Fire, Ruled by Sun

First Decan: 0-10 degrees, ruled by Sun

Second Decan: 10-20 degrees, ruled by Jupiter

Third Decan: 20-30 degrees, ruled by Mars

Virgo: Earth, Ruled by Mercury

First Decan: 0-10 degrees, ruled by Mercury

Second Decan: 10-20 degrees, ruled by Saturn

Third Decan: 20-30 degrees, ruled by Venus

Libra: Air, Ruled by Venus

First Decan: 0-10 degrees, ruled by Venus

Second Decan: 10-20 degrees, ruled by Uranus

Third Decan: 20-30 degrees, ruled by Mercury

Scorpio: Water, Ruled by Pluto

First Decan: 0-10 degrees, ruled by Pluto

Second Decan:10-20 degrees, ruled by Neptune

Third Decan: 20-30 degrees, ruled by Moon

Sagittarius: Fire, Ruled by Jupiter

First Decan: 0-10 degrees, ruled by Jupiter

Second Decan: 10-20 degrees, ruled by Mars

Third Decan: 20-30 degrees, ruled by Sun

Capricorn: Earth, Ruled by Saturn

First Decan: 0-10 degrees, ruled by Saturn

Second Decan: 10-20 degrees, ruled by Venus

Third Decan: 20-30 degrees, ruled by Mercury

Aquarius: Air, Ruled by Uranus

First Decan: 0-10 degrees, ruled by Uranus

Second Decan: 10-20 degrees, ruled by Mercury

Third Decan: 20-30 degrees, ruled by Venus

Pisces: Water, Ruled by Neptune

First Decan: 0-10 degrees, ruled by Neptune

Second Decan: 10-20 degrees, ruled by Moon

Third Decan: 20-30 degrees, ruled by Pluto"

source: https://www.tarot.com/astrology/decans