Traditionally, the Hebrew letter Shin is inscribed on the mezuah, a vessel which houses a scroll of parchment with the Shema (Deut. 6:4-9) text written on it. The Shema Yisra'el (Hear O Israel), calls the children of Israel to love their God with all their heart, soul and strength and to write God's commandments on their hearts. The shape of the letter Shin mimics the structure of the human heart: the lower, larger left ventricle and the smaller right ventricle are positioned like the lines of the letter Shin.
Hear, O Israel: The LORD our God, the LORD is one.
You shall love the LORD your God with all your heart and with all your soul and with all your might. And these words that I command you today shall be on your heart. You shall teach them diligently to your children, and shall talk of them when you sit in your house, and when you walk by the way, and when you lie down, and when you rise. You shall bind them as a sign on your hand, and they shall be as frontlets between your eyes. You shall write them on the doorposts of your house and on your gates. Deut. 6:4-9
The Mezuzah is affixed: 1.On the right doorpost as one enters. 2.In a slanted position with top pointed to inside of room. 3.In the upper third of doorpost height (shoulder high). 4.On outer 3.2 inches of doorpost width.