It all sounds horribly complicated in written words but it's actually really easy to understand.
You know that in the course of a year, the Sun travels through 12 distinct constellations on the sky, the so-called zodiac. Your birth sign is the sign that hosted the sun when you were born. It takes about a month for the sun to travel through a sign, that's ca. 30 days.
You can take this chunk of 30 days per zodiac sign and divide it into three parts, each of them 10 days long. That's a decan. Each zodiac sign consists of three such decans.
According to tradition, these decans are ruled by the seven planets that are visible to the naked eye and have been known to humanity since its beginnings: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. This is the so-called Chaldean order because the ancient Chaldeans already recognized that Saturn must be the most distant planet of all - since he moves so slooowly. And the Moon is the closest because it moves so quickly.
Following the Chaldean order, each decan is ruled by a planet.
That's the reason why you have on each minor Thoth card two astrological sigils: the planet that rules the decan associated with the card, and the zodiac sign associated with the card. But how do we know which planet rules with decan/minor card?
Here's where the elements come into play.
Each element is associated with three zodiac signs and one tarot suit:
Fire: Aries, Leo, Sagittarius - Wands
Earth: Taurus, Virgo, Capricorn - Disks
Air: Gemini, Libra, Aquarius - Swords
Water: Cancer, Scorpio, Pisces - Cups
There are three zodiac signs for each elements and nine minor tarot cards (2 to 10) of each suit, and accordingly, there are always three tarot minors associated with each zodiac sign.
The 2,3 and 4 of each tarot suit goes with the cardinal sign of each element. (If you don't remember what the cardinal signs are: they're the first signs of each season, bringing in a strong sense of initiative and purpose, lots of fresh energy.) The cardinal signs are Aries, Cancer, Libra and Capricorn (for spring, summer, autumn and winter respectively). That's very logical: the first cards of each suit also bring a strong element of new fresh energy. (Although there is a tiny problem here imo with the stability of the 4...)
The 5,6 and 7 of each suit goes with the fixed sign of each element/season. (Fixed signs rule the season at its height, undiluted by energies from the season before or afterwards). The fixed signs are Taurus, Leo, Scorpio and Aquarius. They're also the symbols that appear in the traditional World cards. These minors and signs have a strong conservative energy.
The 8,9 and 10 of each tarot suit goes with the mutable sign of each element/season. (Mutable signs are the last sign of each season when the smell of the next is already in the air, showing flexibility and openness for change.) The mutable signs are Gemini, Virgo, Sagittarius and Pisces.
Now we have all our ducks in a row and we can start going by Chaldean order through the minors.
The sequence starts with the first sign of the year - and astrologically, the year starts with the Spring equinox, i.e., with Aries. And it starts with the planet Mars in the 1st decan of the 1st zodiac sign: Mars in the 1st decan of Aries. A burst of energy! After Mars comes the Sun, so the Sun rules the 2nd decan of Aries, and Venus rules the 3rd. That covered our cardinal spring sign. Mars, Sun and Venus rule the 2,3 and 4 of Wands, the beginning of Spring.
Now spring is at its height, and we're in the fixed sign of spring, i.e., Taurus. Taurus rules the 5,6 and 7 of Disks, and we continue our Chaldean order with Mercury. Do you follow? 5 of Disks is a Taurus card, ruled by Mercury. What comes next? Moon in Taurus for the 6 Disks. And then we start the Chaldean order again, with Saturn in Taurus for the 7 Disks.
I always feel that the Saturn cards are difficult not only because Saturn is such a difficult sign, so slow and heavy, but also because it's such a difficult jump from the closeness and intimacy of the Moon to the cold, distant majesty of Saturn.
So there we went through Tarurus. What comes next? The mutable sign of Spring, Gemini, with the Swords suit. Gemini is mutable because while it's still Spring, you can feel Summer knocking on the door. There's change in the air.
8 Swords: Jupiter in Gemini
9 Swords: Mars in Gemini
10 Swords: Sun in Gemini
Can you see? The Chaldean order marches on through the decans and the seasons. Three cardinal decans of each season (2,3,4) - three fixed decans (5,6,7) - and three mutable decans (8,9,10).
If you go through the whole deck, you'll see that the last decan (3rd decan of the mutable Water sign Pisces, the last zodiac sign of the year when Winter is getting ready for Spring) is again ruled by Mars. Meaning that Mars helps us making the move into the new year - he pushes us, so to say, from Pisces to Aries.
Now what is the result of this ticking Chaldean clock through the four seasons - each of them consisting of three zodiac signs - each of these consisting of three deans - for the planets? Each planet rules a number of decans. It turns out that these decans are distributed nicely over the different seasons, elements/tarot suits and numbers.
Each planet rules decans/minors from all four elements and all four seasons, and no number more often than one. It really works out so beautifully.
I recommend strongly that you pick out the cards that go with each planet to get a better feeling for their character and energy. They're in a way a family. I scanned them in groups some time ago. Each group is headed by the major associated with the planet. Have a look.
Here we have Saturn, represented by Universe, with his decan cards:
And Jupiter, Wheel of Fortune:
Next is Mars, the Tower: (with six minors, it was a bit difficult to scan)
And the Sun:
Venus, the Empress:
Mercury, the Magician:
and the Moon, High Priestess:
I don't want to get into details now but you can see for yourself how the planet's shapes and colours influence the feeling of each card.
I hope you can also see why people who love Thoth do so - once you understand the underlying esoteric principles, these cards are very logical and connect us to the eternal astronomical cycles - the zodiac (who is passing above your head all the time, visibly at night and invisibly in the daytime), the seasons.
You don't have the narrative hints of the RWS scenic pips but I don't believe that makes it more difficult to read with Thoth - just the opposite. Just think of the many associations a minor card has - check your birthday and in which decan you were born, and when it appears in a reading, it means something to you! Knowing that Jupiter rules Libra in the third decan evokes the Wheel of Fortune and Adjustment/Justice - they're present in this card, though hidden.
If you have a lot of Venus-ruled cards, the reading will have a totally different character from a Saturn-heavy reading.
Thoth-based decks work with the underlying principles, and the better you know them, the easier it is to find the information each card gives relating to a specific question.
