Adjacent Eruvin — Eruvin 60a
תלמוד בבלי מסכת עירובין דף ס/א
[[דף נט/ב: והאמר רב נחמן אמר שמואל אנשי חצר ואנשי מרפסת ששכחו...
...ולא עירבו אם יש לפניהם דקה ארבעה אינה אוסרת ואם לאו אוסרת הכא במאי עסקינן בדלא גבוה מרפסת עשרה ואי לא גבוה מרפסת עשרה כי קא עביד דקה מאי הוי במגופפת עד עשר אמות דכיון דעביד דקה איסתלוקי איסתלוק ליה מהכא
In the absence of a dakah — a low opening, but at least four tefachim high, in front of an alley that signifies its separation from its environs — the members of an alley that did not participate in an eruv forbid the members of the adjacent alley who did make an eruv from carrying. [The Acharonim discuss the question of whether a Tzuras HaPesach suffices in this regard.]
For this reason, one cannot make an eruv for half a city (i.e., multiple eruvin in a single city — see above, 59a-b). There is a dispute as to the actual Halachah:
- Rashi, Rosh, Maharam MeRotenburg — This is only the case with a private city that became a public city — since they originally had on unified eruv they may not now split into two distinct eruvin, unless each alley has a dakah. However, a city that was always public may make separate eruvin with lechayayim or koros (pillars or beams at the edge of each alley).
- Ramban, Rashba — Even a city that was always public may not make separate eruvin with lechayayim or koros. However, this is only the case where the distinct eruvin are adjacent one to another and the lechayayim or koros are only dividing one area in which carrying is permitted from another area in which carrying is permitted. On the other hand, to set off an areas in which carrying is permitted from an area in which carrying is not permitted, lechayayim or koros do suffice.
[A public city that does not include a Reshis HaRabbim (viz., its main road has only one exit) has more lenient Halachic guidelines that a public city that does include a Reshus HaRabbim.]