A mezuzah, Hebrew (מזוזה” (doorpost,” or (ותֹמזוז ( mezuzot in the plural, is a decorative case with piece of parchment called a klaf (or qelaf, קלף (inside. The decorative case will likely have the Hebrew letter Shin written upon it, the first letter of one of God’s names, Shaddai (Hebrew: Almighty). The paper, the kalf, is inscribed with specific Hebrew verses from the Torah. These verses consist of the Jewish prayer, Shema Yisrael. This prayer is Deuteronomy 6:4–9 and 11:13-21.
Sometimes Numbers 15:38-41 may be added at the end as well.
The prayer is inscribed on one side of the parchment. On its reverse side of the paper “Shaddai” is written.
Many Jews, and now even Christians, will affix a mezuzah to the doorpost of their homes to fulfill the mitzvah to “write the words of God on the gates and doorposts of your house” (Deuteronomy 6:9). Some will put a mezuzah in every doorway in their home, except closets, laundry rooms, and bathrooms, etc. as these are not living spaces.
See 5 Moses 2:11; Deuteronomy 6:9, 11:20.