Eruvin 5B

פחות מארבע אמות ⁠— נידון משום לחי, ואין צריך לחי אחר להתירו. ארבע אמות ⁠— נידון משום מבוי, וצריך לחי אחר להתירו.

if its protrusion is less than 4 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 4 cubits, that section is deemed an alleyway, and another side post is required to render it permitted to carry in it.

אותו לחי, היכן מעמידו? אי דמוקי ליה בהדיה, אוספי הוא דקא מוסיף עליה.

The Talmud poses a question: That side post, which is added in order to permit carrying within the alleyway that was formed by the 4-cubit side post, where does one position it such that one may carry within the alleyway? The Talmud 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.

אמר רב פפא: דמוקי ליה לאידך גיסא. רב הונא בריה דרב יהושע אמר: אפילו תימא דמוקי לה בהדיה דמטפי ביה, או דמבצר ביה.

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.

אמר רב הונא בריה דרב יהושע: לא אמרן, אלא במבוי שמונה. אבל במבוי שבעה ⁠— ניתר בעומד מרובה על הפרוץ.

The Talmud limits the application of Rami bar Ḥama’s ruling: Rav Huna, son of Rav Yehoshua, said: We stated this halakha with regard to a side post protruding 4 cubits into the alleyway only in the case of an alleyway that is at least 8 cubits wide. However, in the case of an alleyway that is only 7 cubits wide, it is permitted to carry within the alleyway without an additional side post, not because the original side post functions as a side post but rather because it sufficiently seals off the entrance to the alleyway such that the standing segment is greater than the breached segment. The alleyway is now closed off from all 4 sides, and the remaining opening to the public domain is regarded as an entrance, as most of that side is closed and only a small part of it is open.