When the Sefirot are first presented in the Tanya (the classic text from the first Lubavitcher Rebbe) they are called Three Mothers and Seven Doubles. The Three Mothers refer to the three higher Sefirot Keter, Chochmah and Binah (Crown, Wisdom and Understanding). The Seven Doubles refer to the seven lower Sefirot Chesed, Gevurah, Tiferet, Netzach, Hod, Yesod and Malchut (Love, Strength, Beauty, Victory, Majesty, Foundation and Kingship).
Sefer Yetzirah teaches that there are
Three Mother Letters (Primary Letters) -
Aleph א, Mem מ and Shin ש.
From Sefer Yetzirah:
"Three Mothers Letters Aleph, Mem and Shin
are Air, Water and Fire.
Fire (Shin )is above, Water (Mem) is below,
and the Breath of Air (Aleph) balances between them.
The following quote is from the very beginning of Sefer Yetzirah.
These are the Hebrew words that can be seen surrounding
the Tree of Life diagram in this picture:
"With Thirty-two Mystical Paths of Wisdom
God (Yah...) engraved and created His Universe
with Three Sepharim:
with Sefer (letters),
with Sephar (numbers)
and with Sippur (a story).
The Seven Double Letters are Bet ב, Gimel ג, Dalet ד, Kaf כ, Peh פ, Resh ר, and Tav ת.
They direct themselves with two pronunciations.
Their structure is thus both soft and hard, both strong and weak.
The Seven Double Letters Bet, Gimel, Dalet, Kaf, Peh, Resh, Tav -
their foundation is Wisdom, Wealth, Seed, Life, Dominance, Peace and Grace.
The Seven Double Letters are in speech and its opposite.
The opposite of Wisdom is Folly,
Three Mother Letters are Mem מ, Shin ש and Aleph א.
Their foundation is the (right) hand of merit, the (left) hand of liability,
and the tongue of decree that balances between them.
Three Mother Letters Aleph Mem Shin are a great mystical secret,
covered and concealed with six rings.
Out of them came Air, Water and Fire.
From them are born Fathers.
And from the Fathers, descendants.
Three Mother Letters Aleph Mem Shin -
Twenty-two Foundation Letters:
Three Mother Letters,
Seven Double Letters
and Twelve Simple Letters.
The Three Mother Letters are Aleph א, Mem מ and Shin ש.
Their foundation is the pan of merit, the pan of liability,
and the tongue of decree deciding between them.
Mem hums, Shin hisses.
And Aleph is the Breath of Air that is balanced between them.
Twenty-two Foundation Letters -
He engraved them,
This picture shows the kabbalistic Tree of Life diagram with the names of the Sefirot written in Hebrew around the circles. The lines connecting the circles have the Twenty-two Hebrew letters written upon them.
Notice how if you look at any six circles that form a hexagon in this picture - with the seventh circle in the center of the six - a three-dimentional cube can be seen. This is similar to the visual effect seen in my picture The Holy Palace.