Q. Why is the Mezuzah placed on the doorpost slanted?
A. In Ashkenazi tradition, the top of the mezuzah is inclined towards the inside of the room and the bottom towards the outside. How did this come about?
In his monumental compendium on Jewish law - the Arba Turim - Rabbi Jacob ben Asher (13th-14th centuries) cites two conflicting opinions. He first quotes Rashi who taught the mezuzah should be placed vertically. He then brings the view of Rashi's grandson, Rabbeinu Tam, who opined that placing the mezuzah in "standing" position is not respectful. Rather, he taught that it should be placed horizontally, similar to how the Tablets and Torah scroll were arranged in the Holy Ark in the Temple.
Think of the situation in the synagogue when the Sefer Torah is removed from its ark: All the time that it is being held up vertically, all the people stand. As soon as it is placed down to rest, the people also may sit. Rabbeinu Tam seems to be saying the same about the mezuzah.
So what are we to do? Rabbi Jacob ben Asher concludes that those who are careful to do the mitzvahs in the best way possible fulfill both opinions (at least partially) by placing their mezuzahs on a slant.
In his glosses to the Code of Jewish Law, Rabbi Moshe Isserles writes that the position of Rabbi Jacob ben Asher's "careful ones" has since become prevalent in Ashkenazi communities. In Sepharadic communities, however, people follow Rashi's opinion, affixing their mezuzahs vertically.
To find out everything else you'd like to know about mezuzahs, see our mezuzah mini-site.