inscribe them on the doorposts of your house and on your gates.

That is that He commanded us to make a mezuzah. And that is His, may He be exalted, saying, "And you shall write them on the doorposts of your house and on your gates" (Deuteronomy 6:9). And note that this command was repeated; and they have already explained the laws of this commandment in the third chapter of Menachot. (See Parashat Vaetchanan and Ekev; Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 5.)

How is a mezuzah written? The two portions, Shema and V'hayah im shamo'a, are written on one piece of parchment in a single column. Approximately half a fingernail of space should be left above and below [the writing].
Should one write [a mezuzah] in two or three columns, it is acceptable, as long as it not written tail-shaped, in a circle, or tent-shaped. If it was written using any of these forms, it is not acceptable.
If it was not written in order - e.g., one wrote the passage [V'hayah im shamo'a] before the passage [Shema] - it is not acceptable. If one writes a mezuzah on two different parchments, it is not acceptable even if they were sewn together [later].
A mezuzah should not be made from a Torah scroll or tefillin that have become worn, nor should a mezuzah be written on the empty parchment from a Torah scroll, because one should not lower an article from a higher level of holiness to a lesser one.

It is a common custom to write [God's name,] Shaddai, on the outside of a mezuzah opposite the empty space left between the two passages. There is no difficulty in this, since [the addition is made] outside.
Those, however, who write the names of angels, other sacred names, verses, or forms, on the inside [of a mezuzah] are among those who do not have a portion in the world to come. Not only do these fools nullify the mitzvah, but furthermore, they make from a great mitzvah [which reflects] the unity of the name of the Holy One, blessed be He, the love of Him, and the service of Him, a talisman for their own benefit. They, in their foolish conception, think that this will help them regarding the vanities of the world.

When [a mezuzah] is folded, it should be rolled from the end of the line to its beginning so that when a reader rolls it open, he will be able to read from the beginning of the line to the end.
After rolling it, one should place it in a tube made of reed, wood, or any other substance and affix it to the doorpost of one's entrance with a nail. Alternatively, one should hollow out the doorpost and place the mezuzah within.

Before affixing it on the doorpost of the entrance, one should recite the blessing: "Blessed are You, God, our Lord, King of the universe, who has sanctified us with His commandments and commanded us to affix a mezuzah."
One should not recite a blessing when writing [the mezuzah], because affixing it fulfills the mitzvah.

A mezuzah [placed] on private [property] should be checked twice in seven years, and a mezuzah [placed] on public [property] should be checked twice in fifty years, lest a letter have become torn or faded. Since it is affixed within a wall, there is the possibility that it will decay.

Everyone is obligated [to fulfill the mitzvah of] mezuzah, even women and slaves. Minors should be educated to affix a mezuzah to [the doors of] their homes.
A person who rents a dwelling in the diaspora, and a person who rents a room in a hotel in Eretz Yisrael, are exempt from the obligation [to affix a] mezuzah for thirty days. One who rents a house in Eretz Yisrael, however, is obligated [to affix a] mezuzah immediately.

There are ten requirements that must be met by a house for the person who dwells within to be obligated to affix a mezuzah. If one of the requirements is lacking, there is no obligation for a mezuzah. They are:
a) for the area [of the dwelling] to be four cubits by four cubits or more;
b) for it to have two doorposts;
c) for it to have a lintel;
d) for it to have a roof;
e) for it to have doors;
f) for the entrance to be at least ten handbreadths high;
g) for the dwelling not to be consecrated;
h) for it to be intended for human habitation;
i) for it to be intended to be used for a dignified dwelling;
j) for it to be a permanent dwelling.

[The gates to] the Temple Mount, its chambers, courtyards, and, similarly, entrances to synagogues and houses of study which do not have apartments in which people live do not require mezuzot, because they are consecrated.
A synagogue in a village in which guests reside requires a mezuzah. Similarly, a synagogue in a metropolis, if it has an apartment, requires a mezuzah.
All the gates in the Temple complex did not have mezuzot, with the exception of the Gate of Nicanor and those further within, and the entrance to the Chamber of Parhedrin, because this chamber served as a dwelling for the High Priest during the seven days when he was separated [from his home in preparation for the Yom Kippur service].

A toilet, a bathhouse, a mikveh, a tannery, and the like, do not require a mezuzah, since they do not constitute a dignified dwelling.
A sukkah on the holiday of Sukkot, and a house on a ship do not require a mezuzah, for they do not constitute a permanent dwelling.
[With regard to] the two booths of a potter, one inside the other: The outer booth does not require a mezuzah, because it is not a permanent structure. Stores in a market place do not require a mezuzah because they are not permanently used as a dwelling.

Where is the mezuzah affixed? At the inside of the entrance, within a handbreadth of the outer edge of the doorpost, at the beginning of the top third of the entrance. If it was affixed higher up, it is acceptable as long as it is at least a handbreadth below the lintel.
It must be placed at the right-hand side as one enters the house. If it is placed on the left-hand side, it is invalid.
A house belonging to partners requires a mezuzah.

The mezuzah should be erect lengthwise on the length of the doorpost of the doorway, and one should intend that the [word] shema that is at the end of the scroll should be [facing] the outside. Rema: And that is how they practice, but there are those who say that [the mezuzah] is invalid erect, rather its length needs to be inclined towards the width of the doorpost of the doorway [that is to say, horizontally]. And those who are scrupulous fulfill both [opinions] and place the mezuzah at an incline, at a diagonal. And this is how it is proper to practice, and this is how we practice in these places, and one should intend that the top of the mezuzah, where the [word] shema is, should be towards the inside and the last line should be facing outside.

A person must show great care in [the observance of the mitzvah of] mezuzah, because it is an obligation which is constantly incumbent upon everyone.
[Through its observance,] whenever a person enters or leaves [the house], he will encounter the unity of the name of the Holy One, blessed be He, and remember his love for Him. Thus, he will awake from his sleep and his obsession with the vanities of time, and recognize that there is nothing which lasts for eternity except the knowledge of the Creator of the world. This will motivate him to regain full awareness and follow the paths of the upright.
Whoever wears tefillin on his head and arm, wears tzitzit on his garment, and has a mezuzah on his entrance, can be assured that he will not sin, because he has many who will remind him. These are the angels, who will prevent him from sinning, as [Psalms 34:8] states: "The angel of God camps around those who fear Him and protects them."
Blessed be God who offers assistance.