As part of our teshuvah preparations for the new year to ensure that we are fulfilling mitzvot properly, it’s a common custom to have one’s tefillin and mezuzot checked during the month of Elul. In fact, according to some opinions, tefillin and mezuzot should be checked every twelve months. Checking mezuzot entails not only ensuring that our mezuzot are kosher, but also that the place where the mezuzot are affixed is in accordance with halachah. If the mezuzot are not in their proper places, it is as if one is not fulfilling the mitzvah at all, chas veshalom. The mezuzah must be affixed to the right doorpost as one enters the doorway. However, there are complex halachot regarding which is considered the right side if two rooms have a shared doorway; in such a case, one should seek the advice of a Rav to ensure that the mezuzah is placed in accordance with halachah. There are many other details regarding placement of mezuzot that are beyond the scope of this paragraph.
Rav Yosef Yeshaya Braun, shlita, member of the Badatz of Crown Heights; 1 Minute Halacha #402, reprinted from crownheightsconnect.com
