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.