Reference for Menachot 66:6

הא בהדיא קתני לה פתח שאחורי הדלת אם יש שם טפח צריך מזוזה אחרת ואם לאו אינו צריך מזוזה אחרת כיצד קתני

But this is expressly stated further on, [thus,] If there was door behind a door and there was a depth of a handbreadth, another mezuzah is necessary, but if less, no other mezuzah is necessary! - This is merely stated as illustrating [the cases mentioned]. A Tanna taught: If a man set up a door-frame of [hollow] reeds, he may cut away a length of reed and place [the mezuzah in the hollow].

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