For the source text click/tap here: Yoma 11
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Since the Torah said, " Write the words about God on the doorposts of your house and on your gates ," we understand that just as the gates of your house are included, so too the gates of your courtyards, provinces and cities. However, just as a house, it should be inhabited. For example, strictly speaking a synagogue requires a mezuzah only of the sexton lives there.
What about such a gate that is straight at the bottom but oval at the top? Rabbi Meir says that it still needs a mezuzah, while the Sages say that it does not.
All these discussions leads us to an exploration of thresholds and liminal spaces in hilchot mezuzah (and in TS Eliot)