Shema Yisrael
The Shema Yisrael prayer also commands the Israelites to write God's commandments on their hearts (Deut. 6:6); the shape of the letter Shin mimics the structure of the human heart: the lower, larger left ventricle (which supplies the full body) and the smaller right ventricle (which supplies the lungs) are positioned like the lines of the letter Shin.