The Sefer Yetzirah teaches that of the Twenty-two Hebrew letters, three are primary.
These are the Three Mother Letters - Aleph א, Mem מ and Shin ש.
Shin ש is in the triangle in the lighter red. The sound of Shin is an outward, chaotic, hissing sound. Shin is a vertically oriented letter that is open upward - like Fire that rises to the Heavens. Our orientation to the Heavens is vertical.
Mem מ is in the rectangle in the lighter blue. The sound of Mem is an inward, harmonic humming sound. Mem is a horizontally oriented letter that is open downward - like Water that flows down, and how planet Earth is primarily Water. Our orientation to Earth is horizontal.
Aleph א is in the circle. It is a silent letter and represents the Breath of Air that forms all spoken letters. Breath moves in two directions - out and in. The yellow diagonal part of this Aleph is between the upper red part with vertical lines (representing the exhale Shin) and the lower blue part with horizontal lines (representing the inhale Mem).
Hebrew letters are also numbers. This is called gematria (number value). The word gematria is very similar to the word geometry. Thus the Three Mother Letters can also represent the three primary geometric shapes - triangle, circle and rectangle.
Shin is the 21st Hebrew letter. Two plus one equals three thus the gematria of Shin is three - and a triangle has three sides. Mem is the 13th letter. One plus three equals four thus the gematria of Mem is four - and a rectangle has four sides. Aleph is the first letter and thus its gematria is One. In Sefer Yetzirah it says, “One is the Breath of the Living God”. When we say Hashem Echad השם אחד (The Name of God is One) we should mean Infinite and all-inclusive, like a circle, which has an infinite number of sides.
Notice how Shem שם (Name) is spelled with Shin ש and Mem ם, and Aleph א is One. Also notice how the yellow diagonal of the Aleph is the diameter of a circle. In geometry, the ratio between the circumference of a circle and its diameter is pi (3.14.….) - an irrational number that goes to infinity. The first three digits of pi are three (like the triangular Shin); one (like the circular Aleph); and four (like the rectangular Mem)!