Back in the alleyway (מבוי) TB Eiruvin 5

Today’s daf TB Eiruvin 5 provides us with basic information about the koreh (קורה), the crossbeam, and the lekhi (לחי), the side post, which rabbinically fixes the alleyway (מבוי) so that a person may carry within it.

Because of a disagreement between Rav Yosef and Abaye we learn of the two different possibilities the koreh serves. One possibility is that the koreh serves as a conspicuous reminder (משום היכר) of the border between the alleyway and the public domain. The other possibility is that the koreh serves as a fourth wall (משום מחיצה) to enclose the alleyway. “this Master, Rav Yosef, holds that a cross beam functions in an alleyway as a conspicuous marker that demarcates the alleyway from the public domain, and consequently a mere handbreadth is sufficient, as even a handbreadth is sufficiently conspicuous. And this Master, Abaye, holds that a cross beam serves as a partition, and a partition is not effective for an area of less than four handbreadths. The principle that an outer edge descends and seals the alleyway does not apply if the beam is higher than twenty cubits. In order for it to be considered a partition, there must be at least four handbreadths that are less than twenty cubits beneath the cross beam.” (Sefaria.org translation)

 There is a practical halakhic difference between which edge forms that “fourth” wall. If the wall begins at the inner edge, one may carry up to but not under the koreh. If the wall begins at the outer edge, one may carry under the koreh. Rambam poskins that the purpose of the koreh is as a conspicuous reminder. (Mishneh Torah, Zemanim, Shabbat, 17:9 (15) I believe that means one may carry under the koreh if this type of alleyway having three walls formed by at least two courtyards and two houses in each courtyard (מבוי) exists in our day.

The Gemara ponders whether a fourth wall more than four amot protruding from the side of the alleyway serve as a lekhi. “The Gemara examines Rami bar Ḥama’s statement cited in the course of the previous discussion. As to the matter itself: Rami bar Ḥama said that Rav Huna said: With regard to a side post that protrudes from the wall on the side of an alleyway into the entrance of the alleyway, if its protrusion is less than four cubits, it is deemed a side post that renders it permitted to carry in the alleyway, and no other side post is required to render it permitted. However, if it protrudes four cubits, that section is deemed an alleyway, and another side post is required to render it permitted to carry in it.” (Sefaria.org translation)

 The Gemara asks a follow-up question. “The Gemara poses a question: That side post, which is added in order to permit carrying within the alleyway that was formed by the four-cubit side post, where does one position it such that one may carry within the alleyway? The Gemara clarifies its difficulty: If one positions it alongside the first side post as an addition to it, it looks as if he is merely extending the original side post, and it is not noticeable that an extra side post is present. Rav Pappa said: He should position it, the extra side post, on the other side of the alleyway, near the opposite wall. Rav Huna, son of Rav Yehoshua, said: Even if you say that he positions it alongside the first side post, it is valid, so long as he adds to it or diminishes from it in thickness or height, so that it will be noticeable that it is a side post of its own.”