I made a poster version of this picture in the early 1990's which has sold out so I finally reproduced it and made it available as the high quality Giclee print. The red Shin ש and the blue Mem מ boldly show their triangular and rectangular forms - Shin, the 21st Hebrew letter (2+1 = 3 sides of a triangle); Mem, the 13th Hebrew letter (1+3 = 4 sides of a rectangle).
In this picture you can see the Three Mother Letters of Sefer Yetzirah:
See the large Hebrew letter Aleph א?
Now look at the yellow diagonal line. It can be seen as the Hebrew letter Vav pronounced Wow. It can also be seen as the numeral One. The Wow of this Aleph balances and united the red Shin ש the blue final Mem ם.
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.
This picture can used as a focus for meditation. Sit comfortably and become aware of your chest rising and falling with your breath. Now breathe in through your nose down into your belly with your lips closed. This is the humming sound of the blue letter Mem in the lower part of this picture.