Eruvin 87B

הכא רשויות דרבנן.

However, here, Rav Dimi is referring to domains by rabbinic law. As the transfer of objects from a private domain to a karmelit is prohibited only by rabbinic decree, the rabbis did not prohibit this transfer when it is accomplished by way of an exempt domain.

והא רבי יוחנן ברשויות דרבנן נמי אמר, (דתניא): כותל שבין שתי חצירות, גבוה עשרה טפחים ורוחב ארבעה ⁠— מערבין שנים, ואין מערבין אחד.

The Talmud raises a difficulty: But R' Yoḥanan said that transferring objects from one domain to another by way of an exempt domain is prohibited even in the case of domains that apply by rabbinic law. As we learned in a Mishnah: In the case of a wall that is between two courtyards, if it is 10 handbreadths high and 4 handbreadths wide, the residents establish two eiruvin, a separate one for each courtyard, but they do not establish one joint eiruv.

היו בראשו פירות ⁠— אלו עולין מכאן ואוכלין, ואלו עולין מכאן ואוכלין.

If there was produce on top of the wall, these, the residents of one courtyard, may ascend from this side and eat them, and those, the residents of the other courtyard, may ascend from the other side and eat them, provided that they do not bring the produce down from the top of the wall to the courtyards.

נפרץ הכותל, עד עשר אמות ⁠— מערבין שנים, ואם רצו מערבין אחד, מפני שהוא כפתח. יותר מכאן ⁠— מערבין אחד ואין מערבין שנים.

If the wall is breached, a distinction applies: If the breach is up to 10 cubits wide, they may establish two eiruvin, and if they wish, they may establish one eiruv, for it is like an entrance. This breach is similar to any opening of less than 10 cubits. If the breach is more than this, they may establish one eiruv, but they may not establish two eiruvin. A breach of this size nullifies the partition, as the two courtyards merge into a single domain.