Why is the mezuzah slanted?

The mitzvah

“You shall write them on the doorposts (mezuzot) of your house and on your gates” (Deuteronomy 6:9). A mezuzah is a parchment with the first two paragraphs of the Shema, handwritten by a scribe, rolled and placed in a case on the doorpost.

The dispute

The Talmud says a mezuzah placed “like a bolt” is invalid. Rashi understood it should stand vertically; Rabbeinu Tam held it should lie horizontally. The Shulchan Aruch rules vertical. The Rema writes that the meticulous fulfil both views by placing it diagonally, with the top towards the inside, and that is the Ashkenazi custom.

Placement

On the right side of the entrance as one enters, at the start of the upper third of the doorpost, within a handbreadth of the outer edge. Every doorway of a living space needs one, except bathrooms.

Sephardi practice

Sephardim and Jews of Middle Eastern origin, following the Shulchan Aruch, fix the mezuzah vertically. A family should follow its own tradition; a mezuzah in either position is valid.

Affixing it

The mezuzah is fixed with nails, screws or strong tape, not leaned or hung; on fixing it one says the blessing “likboa mezuzah”. A rented home outside Israel is exempt for the first thirty days; an owned home, or any home in Israel, gets its mezuzah right away. It is checked by a scribe twice in seven years, since letters can fade or crack.

What makes it valid

Only a handwritten parchment by a qualified scribe is a mezuzah; a printed copy is a decoration. Buy from a reliable source.