Q. Why are Mezuzahs placed at a slant?

A. Actually, it is an old argument between Rashi and Rabbeynu Tam (both French Rabbis at start of the Millennium) if they should be paced vertically or horizontally at the doorpost. The consensus is (and therefore the Halacha) that we follow Rashi’s opinion and affix them vertically. In order to partially fulfill Rabbeynu Tam’s opinion, we slant it a bit towards the entry. Many Sephardic Jews do not slant it, as they follow Rabbi Josef Karo’s (early 1600’s in Safed) ruling like Rashi, without the deference to Rabbeynu Tam’s ruling.