Hello and welcome! Today, we're gonna learn about decans or decants. From the word "decan" which means 10, and from what we know, a sign takes up 30° of the whole cycle.
Decants
So here, we separate the 30° into tens; hence, resulting in 3 different groups of the same horoscope sign. This is also another method to explain why people of the same signs have different personalities. Now, let's start... *drums rolling*
Okay, this part is pretty simple, you divide one sign into 3, for example;
Aries' date ranges from 21 May till 20 April, now divide it into three categories, each one has at least 10 days. So,
21 May - 30 May is the first decant
1 April - 10 April is the second decant
11 April - 20 April is the third decant

Keep doing that until the last one, Pisces. And now, time for interpretation. As you can see, in each element there are 3 signs(like previously shared on Part II [CLICK HERE])

Fire : Aries, Leo, and Sagittarius
Earth : Taurus, Virgo, Capricorn
Air : Gemini, Libra, Aquarius
Water : Cancer, Scorpio, Pisces

Now, in the first decant, the sign will have the most effect of the horoscope; hence, they're are most likely to act more like it. For example;

First decant of Leo will act more Leo-like, while the second decant will act slightly less like Leo but a bit like Sagittarius, and the last decant is the least to act like a Leo due to co-ruler plant from Aries.

As you can see, the first decants have the most accurate characteristics (because their ruling planet and their co-ruling planet are the same). Meanwhile, the second decant is slightly different from the usual Leo (because their co-ruling planet is the planet that rules the next sign of the same element after the sign Leo). And lastly, the last decant has least traits of Leo (because its co-ruling planet is the planet that rules the last sign of the similar element after the sign Leo). This is a reason why some people have different traits of the usual signs.
For an easier understanding, the first decants are mostly accurate based on the usual characteristics, while the second decant, you have a slight difference from the usual sign. Lastly, the last decant has the least characteristics from the usual signs.
This also relates to cusp (still trying to understand it), for example; the usual Aries would be temperamental, but the Aries-Taurus cusp are calmer than the usual due to Taurus co-ruling planet, and the third decant of Aries is least likely the usual Aries. This shows that decants and cusp might actually be similar, but, I still need to understand this hypothesis of mine a little bit more.
And here, I shall end thy post (so old). Ok, the reason I stop the teachings here is because I still couldn't use, read, or even draw the birth chart. It's pretty accurate, even more accurate than just learning these categorization. BUT, to get accurate results you will need to learn more than just the signs. You'll have to learn the calculations, lines and shapes formed which will be used in the birth chart, and of course, reading the birth chart is related to what I've shared so far. Anyways, thanks for the patience and time given to read and learn together. You can learn the birth chart on your own if you want via Internet. Thanks again, and here is a LINK for you to get your birth chart and you can also learn more from it.