"And thou shall write them upon the doorposts of thy house and on thy gates." That is the commandment found in the Book of Deuteronomy, verse 6, line 9. That is the reason for the mezuzah, the item affixed -- at an angle -- to the doorpost of a Jewish home.
The Hebrew word 'mezuzah' actually means doorpost, but over time it has evolved to mean the doorpost and what is affixed to it. Very little about this important object has been left to chance -- including how it is hung.
You might ask, just what is it that makes a mezuzah so important, and why is it hung on the doorpost, and if it is so important, why isn't it hung straight?
What makes it important is the concept that the doorpost is the dividing line between the swirl of the outside world and the sanctity and safe haven of the home.
Contained in the mezuzah is a tightly rolled piece of parchment made from the skin of a ritually clean animal on which are handwritten, traditionally in twenty-two lines, words from Deuteronomy, the fifth of the Five Books of Moses. Specifically, they are Chapter 6, verses 4 through 9 and Chapter 11, verses 13 through 21, and begin, "Hear, O Israel, the Lord our God, the Lord is One." The parchment, or klaf, is rolled from end to beginning, so that the first word, 'shema,' is on top.
On the back of the parchment is the Hebrew word 'shaddai,' one of the mystical names for the Almighty. Shaddai is also an acronym in Hebrew for Shomer Daltot Yisrael, Guardian of the Gates of Israel.
The mezuzah case should have an opening through which the word 'shaddai' is visible. If the casing is made of a material that does not allow for a window, such as stone, then some feel the word 'shaddai,' or the Hebrew letter shin must appear on the face of the mezuzah. The parchment must be checked twice every seven years.
A mezuzah must be attached on the upper third of the right-hand side of the doorway as one enters, no less than one hand-breadth from the top. A blessing precedes the hanging. A building not used as a permanent residence, such as a succah, does not need a mezuzah.
With all that, the mezuzah is more than an item. Maimonides, a great sage who lived during the twelfth century, wrote, "Whenever one enters or leaves a home with the mezuzah on the doorpost, he will be confronted with the declaration of God's unity...and will be aroused from...his foolish absorption in temporal vanities. He will realize that nothing endures to all eternity save knowledge of the Ruler of the Universe."
So that explains why the mezuzah is important and why it is hung on the doorpost. Yet mezuzot are not restricted to the exterior doorways. Observant Jews affix them to every doorway of every room in the house except the bathroom.
Which brings us back to the real question: why aren't mezuzot hung straight?
One of the most famous French rabbis of the twelfth century was Rabbi Solomon ben Isaac, also known as Rashi. His grandson, Rabbenu Tam, felt that mezuzot should be affixed horizontally for the sake of tradition, because the scrolls in their leather cases were originally pushed horizontally into the crevices between the stones around the doorways of homes.
Rashi argued that mezuzot should be affixed vertically, in such a way that the top pointed toward the Almighty. They eventually compromised, and agreed that a mezuzah should be hung on the diagonal, with its top inclined toward the inside. The decision, allowing peace to rein in a Jewish home in twelfth century France, is part of the message of the mezuzah.
The blessing said while hanging a mezuzah:
Transliteration: Barukh atah Adonai, Elohaynu, melekh ha-olam, asher keedishanu b'meetzvotav v'tzeevanu leek'boa mezuzah.
Translation: Blessed are you, Lord, our God, King of the universe, who has sanctified us with God's commandments and commanded us to affix a mezuzah.