# One may put up a mezuzah at night.<ref>Rav Mordechai Eliyahu (comment on Kitzur Shulchan Aruch 11:10), Ben Ish Chai Parashat Ki Tavo Year 2: Halacha 4, Shevet Hakehati 1:277, Mezuzat Baitecha 289:6, Rivevot Ephraim 7:369. </ref>
===Which Is the Right Side?===
# The criteria by which one can determine which side the mezuzah should be used are
## '''From Exempt to Obligated''': Any door that goes from a place that is exempt from mezuzah to a place that is obligated in mezuzah should have the mezuzah on the right side entering into the place that is obligated in mezuzah. For example, the mezuzah on the front door or back door of a house is always placed on the right side going into the house since the house is obligated in mezuzah and the street isn’t. A walk-in closet which is less than 4x4 amot the mezuzah is put on the right side going from the closet into the room since the closet isn’t obligated.<ref> Binyan Tzion 99 writes that a door from a place that isn't obligated in a mezuzah to a place that is obligated in a mezuzah should have a mezuzah on the right side going from the place that is exempt to the place that is obligated. His example is the closet that is less than 4x4 amot which is seen as an entrance from the closet to the room. Also, a front door to the street certainly has a mezuzah from the street to the house. [http://www.yutorah.org/sidebar/lecture.cfm/788640/rabbi-hershel-schachter/the-laws-of-mezuzah/ Rav Hershel Schachter in “The Laws of Mezuzah” (min 34-42)] explained that this is the first factor by which a person determines on which side of the door the mezuzah is placed. </ref>
## '''Entry''': The mezuzah should be put up on the doorpost which is to one's right upon one's entering the house. <ref> Gemara Menachot 33b, Rambam Hilchot Mezuzah 6:12, Shulchan Aruch 289:2, Taz 289:3, Levush 289:2, Aruch Hashulchan 289:5, Kitzur Shulchan Aruch 11:3, Kitzur Shulchan Aruch of Rav Mordechai Eliyahu 11:3. </ref> This criteria applies equally to the outer doors of the house to the rooms inside the house.<ref>Igrot Moshe YD 4:43:4</ref> For example, a room that only has one entrance obviously has the mezuzah placed on the right side going into the room.<ref>Ben Ish Chai (Ki Tavo, Shana Sheniya no. 6) writes that if there’s a room that’s a dead end then it is obvious that we put the mezuzah on the right going into that room. [http://www.yutorah.org/sidebar/lecture.cfm/788640/rabbi-hershel-schachter/the-laws-of-mezuzah/ Rav Hershel Schachter in “The Laws of Mezuzah” (min 34-42)] agreed. Based on the Maharil (responsa 94) regarding courtyards and balconies this rule is obvious. See below. </ref>
## '''Majority of Walking''': If the position isn't determined by entry or by which is used more frequently it can be determined by which room people walk from one room to another; if majority of the times people enter in one direction the mezuzah is placed on the right of that direction.<ref>[http://www.yutorah.org/sidebar/lecture.cfm/788640/rabbi-hershel-schachter/the-laws-of-mezuzah/ Rav Hershel Schachter in “The Laws of Mezuzah” (min 34-42)] holds that the factor of looking at the majority of the way people walk one room to another trumps the rule of the way the door is placed. Aruch Hashulchan 289:8 and Igrot Moshe YD 1:176 agree. Hamezuzah Vehilchoteha 11:12 holds that this factor is to be used after the door rule. Chelkat Yakov YD 161 agrees.</ref>
## '''Doorway''': If the position of the mezuzah cannot be determined based on which direction is used for entry or which room is used more frequently it can be determined by the door. Since the door generally swings into the room (besides the outermost door of the house) the way the door swings can be considered the way one enters the room and the mezuzah is placed on the right side entering the room.<ref>Menachot 33a, Shulchan Aruch YD 289:3, Aruch Hashulchan 289:6</ref> However, if the mezuzah is determined based on the previous factors it doesn't matter which way the door swings.<ref>Mordechai cited by Bet Yosef YD 289, Shach 289:6, Aruch Hashulchan 289:7, Igrot Moshe YD 4:43:3, Hamezuzah Vehilchoteha 11:11</ref>
# Other factors include:
## '''Walking Route''': Generally, the route by which a person walks around their house should be consecutively entering one room to another creating a circuit. For example, if the door of the house leads exclusively to room A which leads to room B and on to room C then the mezuzot should be placed on the right side of the door entering the house from the outside to A to B to C. However, this rule isn't ironclad, rather the first two rules are definitive.<ref>Igrot Moshe YD 4:43:4</ref>
## '''Doubt''': If it is impossible to determine which side to put up the mezuzah based on the criteria of (1) whether the door is used more for entry or exiting, (2) which of the two rooms that the door connects is used more frequently, and (3) the door hinges because the door is used equally for entry and exiting, the two rooms are used equally and there is no door, according to some poskim there is no obligation to put up a mezuzah at all. Some poskim write that one should put up a mezuzah on both sides, but other poskim write that may not do so since it is [Bal Tosif], adding onto a mitzvah.<ref>The Yavetz 70 held that in a case of doubt that can't be resolved should have a mezuzah on both sides. The Maharam Shik YD 287 argues that this would violate the prohibition of adding a mitzvah, Bal Tosif.
* Strict: HaMezuzah Vehilchoteha 11:13 citing Mezuzat Beytecha 289:16 quoting the Chazon Ish and Minchat Yitzchak 1:9 agree with the Maharam Shik. Chelkat Yakov 162 holds that it is a rabbinic form of Bal Tosif.