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.