After one has acquired a good-quality mezuza, there are several halakhot regarding how and where to place it in the doorway of the house.

The mezuza should be affixed to the doorpost, on the right side of the doorway as one enters the room. It should be placed on the bottom of the upper third of the doorframe, but it is valid if it is higher than that, as long as it is at least ten centimeters from the top of the doorpost.

The mezuza can be attached to the doorpost in any manner that holds it firm, such as with nails, screws, or glue.

There is a difference in custom between Ashkenazim and Sephardim about the positioning of the mezuza on the doorpost. According to the Sephardic custom, it should be set vertically along the length of the doorpost. The Ashkenazic custom is to place the mezuza on a slight diagonal, with its upper side tilted toward the inside of the room and the lower side angled outward; see illustration.

Before affixing a mezuza, one recites the following blessing:

“Blessed are You, Lord our God, King of the universe, who sanctified us through His commandments, and commanded us to affix a mezuza.”

When affixing mezuzot in several doorways in the house, one recites the blessing only once, before affixing the first mezuza, and this blessing applies to the other mezuzot as well.

In certain situations, there may be a doubt as to whether there is an obligation to affix a mezuza in a particular doorway, or there may be doubt about its proper location. In such cases, a rabbi should be consulted.

The following are some examples of situations in which there is uncertainty as to the obligation of setting a mezuza or its location: when the lintel on top of the door is round rather than straight; when the door closes on the outside of the lintel rather than under it; when the doorway is particularly high; when there is not enough space for attaching a mezuza inside the doorway.

In all the rooms of the house, the mezuza is affixed on the doorpost to the right side as one enters the room, coming from the entrance of the house. If two rooms are open to each other such that a doorway can be accessed from either side and it is not obvious in which direction one is considered to be “entering” and in which direction one is considered to be “leaving,” the determination is made based on the common flow of traffic. If the flow of traffic is equal in each direction, the mezuza should be affixed on the right side of the doorpost as one pushes the door open from one room to the other. In case of doubt, one should consult a rabbi.

The obligation to affix a mezuza applies to the main entrance of every residence, as well as the doorways to all the internal rooms that are used for residential purposes and whose size exceeds four square meters. This category includes bedrooms, storage rooms, balconies, courtyards, and stairwells.

No mezuza should be placed at the entrance to bathrooms.

The mezuzot on the doors of the house are all significant, but the mezuza that is affixed at the main entrance of the house is of special importance. There are two practical ramifications to this concept:

(1) If one cannot afford to buy superior-quality mezuzot for all the doorways, the mezuza for the main entrance is the most important and, therefore, it is worthwhile to invest in it before the others.

(2) There is a halakhic rule that in matters of holiness “one may raise up but not lower,” i.e., one may promote an object of sanctity from an inferior to a superior position, but not vice versa. Therefore, one should not “demote” a mezuza by moving it from the main entrance of the house to one of its interior rooms.

Mezuzot should be taken down to be examined every three and a half years. Some people have them checked once a year, in the month of Elul. The examination should be performed by one who is well versed in the relevant halakhot.

If the mezuzot were found to be kosher, there is disagreement between halakhic authorities about whether to recite the blessing “to affix the mezuza” when returning them to their places. Each person should follow the custom of his community and the instruction of his rabbi. If a mezuza was disqualified and replaced, all agree that one must recite a blessing when affixing the new mezuza.

Further reading: The full content of the passages of a mezuza can be found in A Concise Guide to the Torah, pp. 451, 462; for insights of the Sages on these sections, see A Concise Guide to the Sages, pp. 231, 236.