Mezuzah
מְזֻזוֹת בֵּיתֶךָ וּבִשְעָרֶיךָ-(דברים ו,ט) וּכְתַבְתָם עַל
Yoma 10-11
וּכְתַבְתָם
Perhaps the mitzvah is to write the portions of the Torah directly on the door post?
The verse in Yirmiyahu state, ואני כותב על הספר בדיו (and I wrote on the Sefer in ink); just as those words were written on a scroll with ink so here in this mitzvah.
וּכְתַבְתָם
Specific laws apply to the mitzvah of writing a Mezuzah that is different from writing a Sefer Torah.
מְזֻזוֹת
Written without the Vav between the two ziyins; even though a mezuzah requires two door posts to be obligated, one place the written scroll on one door post (by contrast in Egypt written full, the blood was placed on both door posts).
וּכְתַבְתָם
Connected to וקשרתם (from prior verse about Tefillin); since the knotting of the Tefillin is with the right hand; the placement of the mezuzah is on the right side.
בֵּיתֶךָ
Requires two door posts and a roof the minimum qualifications of a house to be obligated in a mezuzah.
זז מות
Mezuzah guards from the angel of death entering the house.
בֵּיתֶךָ
Your house and not you neighbor’s house; different parts of Krias Shema alludes to the Ten Commandments; לא תחמוד not to desire property of others.
וּבִשְעָרֶיךָ
Assists to define ביתיך; a שער is something that encloses an area; also a בית must have a door that enclosing an area.
עַל
Place of the mezuzah is from the outside of the door post.
וּבִשְעָרֶיךָ
Different standards to obligate (than a house); no need for a hinge, roof, can be within the post; since not a place of guarded or clean.
Areas of Halacha
Where does one place the mezuzah on the door?
Menachos 33a: Shmuel says the mitzvah is to place the mezuzah in the beginning of the upper third of the door post.
Rosh: The entire upper third (except for the amah closest to the lintel) is valid for a mezuzah. The statement to place at the beginning of the upper third is to exclude a lower placement.
Rambam: The optimal placement is exactly at the beginning of the upper third of the doorway, no lower and no higher. Placement above is secondarily okay; placement below is not valid.
Rabbi Yosef Karo: Optimally one should follow the opinion of the Rambam. Therefore, one should measure the height of the doorway, divide into three parts and place exactly at the beginning of the upper third.
Other Poskim (Shach) point out that in general, people are accustom to place the mezuzah according to the opinion of the Rosh, within the upper third without exact measurements. Only some very pious Jews appear to be exact like the Rambam.
However, according to the Ariza”l, one should be very exact like the Rambam. One should measure the doorway, divide into three parts, and place one-third down from the height of the doorway (and brings mystical reasons for this placement).
Therefore, all Poskim agree that if one places the mezuzah below the one-third upper portion, one needs to remove the mezuzah and replace within the upper third. Most state it should be done without a blessing since one opinion in the Gemara approves such a placement.
Often Mezuzos are not in the correct position, mostly too high (within a few inches of the lintel).
So the following video is a useful guide to how to affix a Mezuzah.
http://www.youtube.com/watch?feature=player_embedded&v=youT85Czmio
