Reference for Shabbat 17:7

ואם היתה איסקופה גבוהה י' ורחבה ד' הרי זו רשות לעצמה מסייע ליה לרב יצחק בר אבדימי דאמר רב יצחק בר אבדימי אומר היה רבי מאיר כל מקום שאתה מוצא ב' רשויות והן רשות אחת כגון עמוד ברה"י גבוה י' ורחב ד' אסור לכתף עליו גזירה משום תל ברה"ר:

And should you answer that [the reference is to a threshold which] is not four square: surely R. Hama b. Goria said in Rab's name: That which lies within the opening, even if less than four square, requires another stake to permit it!-Said Rab Judah in Rab's name: The reference here is to the threshold of an alley, half of which [threshold] is covered and half uncovered, the covering being toward the inner side: [hence] if the door is open, it is as within; if the door is shut, it is as without.<span class="x" onmousemove="('comment',' This alley was rendered fit for carrying not by a stake but by a beam across its front (v. 'Er. 11b); and it was also furnished with a door or gate at its opening. Now, the threshold referred to here lies in front of the door, while the beam overhead covers the inner half of the threshold. If the door is open (it opened inwards) the whole threshold is counted as part of the alley, and so it is permitted; if it is closed, the threshold is shut out, and even the portion under the beam is forbidden. ');">⁷</span> R. Ashi said: After all, it refers to the threshold of a house, and e.g., where it is covered over with two beams, neither being four [handbreadths wide], and there are less than three [handbreadths] between them, while the door is in the middle: if the entrance is open, it is as within, if shut, it is as without.<span class="x" onmousemove="('comment',' The entrance was covered over from above; if the cover was a single beam four handbreadths wide, everything beneath it, including the threshold, is permitted, as imaginary partitions are assumed to descend from the sides of the beam parallel to the house and enclose the entrance. But this assumption is not made when the beam is less than four in width. Again, when two beams are less than three handbreadths apart, the whole, including the space, is regarded as one, on the principle of labud, providing that there is nothing between them to break their imaginary unity. Now, the reference here is to a threshold in the middle of which the door is set. If this entrance is open, nothing breaks the unity above, and since the width of the two beams plus the space between is four cubits, the threshold is permitted. But if it is shut, the door coming between the two beams above forbids the assumption that they are united, and by corollary, the imaginary existence of partitions; hence the threshold remains forbidden. ');">⁸</span> 'But if the threshold is ten [handbreadths] high and four broad, it is a separate domain.' This supports R. Isaac b. Abdimi. For R. Isaac b. Abdimi said, R. Meir<span class="x" onmousemove="('comment',' Who is the 'others' mentioned as authors of this teaching, v. supra p. 11, n. 3. ');">⁹</span> used to teach: Wherever you find two domains which are really one, e.g., a pillar in private ground ten high and four broad, one may not re-arrange a burden thereon, for fear of a mound in a public domain.<span class="x" onmousemove="('comment',' Of the same size; since such constitutes private ground, one may not move an article from it into the street, and so even when situated in private ground it is also forbidden, lest one lead to the other. ');">¹⁰</span>

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