Talmud - Eruvin 10b עירובין י׳ ב׳

One prepares a board ten handbreadths high with a length of four cubits and stands it lengthwise down the middle of the alleyway, and thereby forms two small alleyways at the entrance to the alleyway, neither of which is more than ten cubits wide.

Alternatively, one can act in accordance with the opinion of Rav Yehuda, as Rav Yehuda said: If an alleyway is fifteen cubits wide, how does one reduce its width? He distances himself two cubits from one of the walls of the alleyway and prepares a board three cubits wide, thereby leaving an opening of only ten cubits.

The Gemara asks: And why must one reduce the width in this manner? One could also prepare a board a cubit and a half wide, and distance himself two cubits, and then prepare another board a cubit and a half wide, leaving the alleyway with an opening of only ten cubits. Apparently, one may conclude from the fact that Rav Yehuda did not suggest this possibility that if the standing segment of a wall is greater than the breached segment only when one combines the standing segments from two directions, i.e., both sides of the breach, it is not considered as though the standing segment were greater.

The Gemara rejects this: Actually, I would say to you that ordinarily it is considered as standing even when one must combine the standing segments on the two sides of the breach. However, it is different here, as the air, i.e., the two cubit opening, of this one side of the far board and the air, i.e., the ten cubit opening, of this other side of the board come together and negate it. Therefore, in this case, the board that is farther from the wall cannot serve to close off the alleyway.

The Gemara suggests: And one could instead prepare a board one cubit wide and distance himself one cubit, and prepare another board of a cubit and distance himself one cubit, and prepare a third board of one cubit, thus ensuring that the open space is not greater than the standing segment on both sides. Apparently, since Rav Yehuda did not suggest this possibility, one may conclude from this that if the standing segment of a wall is equal to the breached segment, carrying in the alleyway is prohibited.

The Gemara rejects this assumption: Actually, I would say to you that ordinarily carrying is permitted in that case. But here it is different, since the air, the opening, on this side of the board and the air, the opening, on that side of the board come together and negate the effectiveness of the board.

The Gemara suggests: And one could distance himself one cubit from the wall, and prepare a board of a cubit and a half, and distance himself another cubit, and prepare another board of a cubit and a half. In this manner, one could diminish the width of the entrance of the alleyway to ten cubits.

The Gemara answers: Yes, it is indeed so; this would work equally as well. But the Sages did not burden him this much, requiring him to prepare two boards where one suffices.

The Gemara raises a new issue: But let us be concerned lest one abandon use of the larger entrance, which is ten cubits wide, and begin to enter the alleyway through the smaller entrance, which has a width of two cubits. This would negate the larger opening’s status as an entrance and render the alleyway unfit for one to carry within it, as it would no longer have an entrance with a side post. Rav Adda bar Mattana said: The presumption is that a person does not abandon a larger entrance and enter instead through a smaller entrance.

The Gemara raises a difficulty: And in what way is this different from the opinion of Rabbi Ami and Rabbi Asi, who maintain that in the case of an alleyway that is breached on its side wall close to its entrance, if the breach is large enough for one to enter through it, carrying in the alleyway is prohibited? There, too, such a breach should not be problematic, as a person does not abandon a larger entrance to enter through a smaller one.

The Gemara answers: There, in the case of Rabbi Ami and Rabbi Asi, the smaller entrance reduces his walking distance. If one approaches the alleyway from the side, the smaller entrance provides a shortcut, and therefore one might enter through it as well. However, here, in the case of the two entrances one two cubits and one ten cubits, it does not reduce his walking distance, as both openings are situated at the front of the alleyway.

The Gemara returns to the issue of the standing segment that is greater than the breached segment. We learned in the Tosefta there, in tractate Kelim: The leather covering of a stool [asla] and its hole join together to complete a handbreadth with regard to ritual impurity imparted by a tent over a corpse. Any person, vessel, or food that is beneath a covering that is at least a handbreadth in size together with a portion of a corpse of at least an olive-bulk becomes ritually impure with impurity imparted by a corpse. The baraita teaches that the leather covering of a stool and its hole combine to complete the measure of a handbreadth.

The Gemara asks: What is the leather covering of a stool referred to in the Tosefta? Rabba bar bar Ḥana said that Rav Yoḥanan said: The leather covering of a bathroom.

The Gemara asks: And how large can the hole be and still combine with the leather covering to complete the handbreadth? When Rav Dimi came from Eretz Yisrael to Babylonia, he said: Two fingers of leather from here, on one side, and two fingers of leather from here, on the other side, and a space of two fingers for the hole in the middle. However, when Ravin came from Eretz Yisrael to Babylonia, he said: A finger and a half of leather from here, and a finger and a half on the leather from here, and a space of a single finger for the hole in the middle.

Abaye said to Rav Dimi: Do the two of you, yourself and Ravin, disagree in principle? Rav Dimi said to him: No, rather this, Ravin’s statement, is referring to the large finger, i.e., the thumb, and this, my own statement, is referring to the small finger, the pinkie, and we do not disagree. Both were describing one handbreadth, which equals the width of four thumbs or six pinkies.

Abaye said to him: This is not so [la’ei]. You disagree, and you disagree with regard to the halakha in a case where the standing segment of a wall is greater than the breached segment only when one combines the standing segments from two directions, i.e., both sides of the breached segment. According to you, this wall is considered as standing, even when one must combine the standing segments from two directions. According to Ravin, if the standing segment on one side of the breach is greater, the wall is considered as standing; however, if the standing segment is greater only after combining the standing segments from the two directions, it is not considered as standing.

Abaye continues: For if it should enter your mind to say that you do not disagree, but simply refer to the same measures by different names, to express his opinion, Ravin should have said as follows: A finger and a third of leather from here, and a finger and a third of leather from here, and a space of a finger and a third for the hole in the middle. In this case, there would still be a handbreadth in total, but each side of leather alone would not be larger than the space in the middle. The fact that Ravin presented a case where the hole in the middle is smaller than the width of the leather on either side indicates that his dispute with Rav Dimi is a fundamental one.

Rav Dimi responds: Rather, what do you wish to say, that we disagree? If so, to express the opinion attributed to me, I should have said as follows: A finger and two thirds of leather from here, and a finger and two thirds of leather from here, and a space of two fingers and two thirds in the middle. This would provide a more striking case where, despite the fact that the breach is much greater than the standing segments on either of its sides, the two standing segments combine together so that the standing segments are considered greater than the breached segment.

Rather, if there is room to say that we disagree, our dispute relates to a different point, and we argue in the case where the breached segment is exactly equal to the standing segment on each side. According to Ravin, it is considered breached; while according to Rav Dimi, it is considered standing.

The Gemara returns to the mishna: If the entrance to the alleyway has an opening in the form of a doorway, then, even if it is wider than ten cubits, one need not diminish its width. The Gemara comments: We find that an opening in the form of a doorway is effective to permit carrying in an alleyway with regard to its width, i.e., when its entrance is more than ten cubits wide, and that a cornice is effective with regard to its height, i.e., when it is more than twenty cubits high.

Traduction : The William Davidson digital edition of the Koren Noé Talmud, with commentary by Rabbi Adin Even-Israel Steinsaltz.

Eruvin 10b explores how to narrow a wide alleyway using boards of specific widths and spacing to keep the opening within permitted eruv measurements.