## '''Entry''': The mezuzah should be put up on the doorpost which is to one's right upon one's entering the room. <ref> Gemara Menachot 33b, Rambam Hilchot Mezuzah 6:12, Shulchan Aruch 289:2, Aruch Hashulchan 289:5, 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>
## '''Usage''': If the door is used equally for entry and exit then if one of the two rooms that it connects is used more frequently then the mezuzah should be put up on the right side going into the room that is used more frequently.<ref>Igrot Moshe YD 4:43:4. He writes that even though the Daat Kedoshim writes that the mezuzah is determined based on the frequency of use of the rooms, he holds that that only is because generally the direction that is used for entry is going to be used more frequently but if this factor conflicts with the factor of entry we follow that one. Hamezuzah Vehilchoteha 11:11 agrees.</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. Hamezuzah Vehilchoteha 11:12 holds that this factor is to be used after the door rule.
The concept of determining the direction of entry based on majority of the ways people enter </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>
## '''Doors to the Outside''': 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. 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>[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>
## '''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>Igrot Moshe YD 1:176 holds that if the direction in which the mezuzah should be put because it isn't used more for entry than exiting, the rooms it connects are used equally, and there is no door, then there is obligation to put up a mezuzah at all. He quotes the Yavetz who says to put up a mezuzah on both posts out of doubt but he disagrees. He also rejects the opinion of the questioner to put up a mezuzah on the side of his choice. See Rav Mordechai Eliyahu's comment to Kitzur Shulchan Aruch 11:4 where he rights that if there is a doubt as to which side the mezuzah should be placed there is a safek as to what one should do. HaMezuzah Vehilchoteha 11:13 citing Mezuzat Beytecha 289:16 quoting the Chazon Ish and Minchat Yitzchak 1:9 conclude that it is forbidden because of Bal Tosif to put up two mezuzot. [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 it was not Bal Tosif since one was only trying to be fulfill the mitzvah according to all opinions similar to wearing Rashi and Rabbenu Tam's tefillin simultaneously.</ref>