Each sign has 3 decans of each 10 degrees.
It’s important to first understand the 4 elements as each decan is strongly linked to the elements or the so called triplicities (just a fancy way of putting it).
That means that Aries (element Fire) has 3 fire decans.
> Aries=Mars, Leo=Sun, Sagittarius=Jupiter
You can see it in the image below of a chart with Aries rising.
1st Decan is Mars, because Aries/Mars comes first in the zodiacal (fire) order.
2nd Decan is Sun, because Leo/Sun comes second in the zodiacal (fire) order.
4rd Decan is Jupiter, because Sagittarius/Jupiter comes third in the zodiacal (fire) order.

Taurus has 3 earth Decans
Gemini has 3 air Decans
Cancer has 3 water Decans
And so on…

How the heck do you memorize that?
Well first of all it’s important to know the astrological order of the signs from Aries to Pisces.
and than divide that into the 4 elements. If you know that Aries is the first fire sign in the zodiac, than Leo is the 2nd fire sign and Sagittarius the 3rd.
Each decan always starts with it’s own rulership, meaning the first 10 degrees of a sign are the most pure and raw to the typical qualities of that sign. So the first 10 degrees of Leo is also the first decan that is ruled by the Sun. what fire sign is next in the zodiac? Sagittarius, so the 2nd decan is Jupiter. Than the 3rd fire sign is Aries which makes the last 10 degrees of the sign Leo, ruled by Mars. It might sound a bit like a soup, but once you get it, it’s very simple.

Sign0°-9° – 10°-19° – 20°-29°
AriesMars/Aries – Sun/Leo – Jupiter/Sagittarius
TaurusVenus/Taurus – Mercury/Virgo – Saturn/Capricorn
GeminiMercury/Gemini – Venus/Libra – Uranus/Aquarius
CancerMoon/Cancer – Pluto/Scorpio – Neptune/Pisces
LeoSun/Leo – Jupiter/Sagittarius – Mars/Aries
VirgoMercury/Virgo – Saturn/Capricorn – Venus/Taurus
LibraVenus/Libra – Uranus/Aquarius – Mercury/Gemini
ScorpioPluto/Scorpio – Neptune/Pisces – Moon/Cancer
SagittariusJupiter/Sagittarius – Mars/Aries – Sun/Leo
CapricornSaturn/Capricorn – Venus/Taurus – Mercury/Virgo
AquariusUranus/Aquarius – Mercury/Gemini – Venus/Libra
PiscesNeptune/Pisces – Moon/Cancer – Pluto/Scorpio

Note : In this table i’ve inserted the Modern rulerships. You can use the traditional rulerships instead if you prefer to work this way.

Why is this helpful?

It’s descriptive of the qualities and characteristics of a planet or angle, falling in a specific degree = Decan in a zodiac.
In the example image above, the ASC is at 0′ of Aries and therefore in the 1st Decan which is ruled by Mars, so in other words a double Aries/Mars energy. If the ASC was at 15′ Aries than it would have been in the Decan of the Sun, adding Leo qualities to the Aries rising person.
The same applies to the Sun in Aries, our sunsign. If a native has their Sun at 5 degrees of Aries, than the native is a true Aries as the Sun is sitting in the Decan of Mars. If a native has their Sun at 27 degrees of Aries than it’s in the Decan of Jupiter, so in other words the native is a Aries/Sagittarius.

Whatever degree, and therefore decan a planet is placed at, is than given a co-ruler.
Such as a Person with Sun in Libra at 16′ has Venus as their usual ruler but as it sits into the Decan of Uranus it’s therefore a co-ruler of the Libra Sun native.

For example, you may meet an Aquarius and as per basic Astrology this sign is often labeled as emotionless or not very romantic. But if this Aquarius native has their Sun at 25′, than it means it’s in the Decan of Venus (Libra). Given them Venusian qualities.

It’s useful for understanding why not every Libra is a typical Libra for example. Just look at the Decan for a more in depth explanation of a planet’s position in your birthchart.

Decan Calculators & External links

Brief explanation – Calculator
Compatibility via Decan (Sunsign) – Calculator

Brief explanation – When you know your Decan
In depth explanation – When you know your Decan