Reference for Menachot 66:5
כי תניא ההיא בפתח שאחורי הדלת
and there was the depth of a handbreadth,<span class="x" onmousemove="('comment',' In the first clause, presumably this means that it was set in deep in the post to the depth of a handbreadth, and in the second clause this means that the thickness of the new frame was a handbreadth, so that the mezuzah on the original frame is now sunken in to the depth of a handbreadth.');"><sup>3</sup></span> another mezuzah is necessary, but if less, no other mezuzah is necessary! - That [first clause of the Baraitha] refers to a door behind a door.<span class="x" onmousemove="('comment',' The post referred to in the first clause of the Baraitha was a post that served as the right door-post for two doors. Thus, through the first door one entered the house, and at right angles to this door and hard by it on the right there opened another door through which one entered into an inner room. If the thickness of this door-post was a handbreadth or more, then two mezuzoth are necessary, but if less, then one mezuzah serves for both doors. Similarly the framework spoken of in the second clause of the Baraitha refers also to this post, thus a jamb was added on each side of this door-post making the thickness of the whole more than a handbreadth. Another explanation is that the Baraitha refers to a small door that is made in a large door; if the width from the right edge of the small door to the right edge of the large door is a handbreadth or more, then each door requires a mezuzah; but if less, one mezuzah (i.e., the one on the doorpost of the large door) serves for both doors.');"><sup>4</sup></span>