423. Knock, Knock: The obligation to put a mezuzah on one’s doors
Write them upon the doorposts of your house… (Deuteronomy 6:9)
“Mezuzah” as we use the term is something of a misnomer. The word literally means a doorpost, but we use it to refer to a small strip of parchment containing particular Biblical verses, which we affix to our doorposts as per this mitzvah. The mezuzah contains two passages, Deuteronomy 6:4-9 (which includes the Shema) and Deuteronomy 11:13-21.
The mezuzah is placed on the right-hand side of an entryway, in the upper third of the doorpost. The accepted practice is to position the mezuzah at an angle with the top portion pointing inward. A mezuzah is placed on pretty much every room except a bathroom. A renter (outside of Israel, at least) does not have to affix a mezuzah to their dwelling for 30 days, so travelers do not have to worry about their hotel rooms. If a person moves out of a house or apartment and the next occupants are Jewish, he must leave the mezuzos for the next tenants (though they are expected to pay for them).
The basis of this mitzvah is to constantly remind us of God and His Torah. We are cognizant of the mezuzah as we enter and exit our homes and it serves as a reminder to keep the mitzvos.
This mitzvah applies to men and women in all times and places. It is discussed in the Talmud in tractate Menachos (31b-34a) and in Yoma (10a-11b). It is codified in the Shulchan Aruch in Yoreh Deah 285. This mitzvah is #15 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #12 of the 77 positive mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.