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Pedestrian Gate Fitted Inside a Car Gate
Q: We built a pedestrian gate into our electric sliding gate for use on Shabbos and Yom Tov. It is a proper door, with posts and lintel. The sliding gate itself has posts, but no lintel. Do I need to put mezuzot on both entrances?
A: Surprisingly, neither entrance needs a mezuzah. The sliding gate is exempt, even though it has posts, because it has no lintel. (If it did have a lintel, one would place a mezuzah on the right post framing the gateway.) 
The pedestrian door, though it is an entrance, is also exempt as it is deemed to be merely part of the larger gate.
 Shulchan Aruch 287:1.
 Agur B’ohalecha 22:8. This is true even according to the dissenting opinion that holds the inner door needs a mezuzah. Since the rarely used outer gate has a lintel, it is the main gate, and its mezuzah exempts the small gate. Yad Haketanah 3:9; Tzemach Tzedek, Piskei Dinim Y.D., cited in Sha’arei HaMezuzah 11:29.
 Aruch HaShulchan 286:45; Minchas Yitzchak 7:12; She’arim HaMetzuyanim B’halachah 11:4. Mezuzas Melachim (cited in Sha’arei HaMezuzah 11:29) reports that the custom is to exempt the inner gate. If the main gate was only used very rarely, some authorities rule that the small gate then becomes the main door and the big gate is deemed to be a wall. In such a case, one would put a mezuzah on the right post of the small gate. However, this would only be true if the main gate did not have a mezuzah. See following note.
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