Eruvin 73B

ואם נשתתפו במבוי ⁠— מותרין כאן וכאן.

And if they merged the courtyards of the alleyway, they are permitted to carry both here, in the alleyway, and there, in the courtyards.

עירבו בחצירות ונשתתפו במבוי, ושכח אחד מבני חצר ולא עירב ⁠— מותרין כאן וכאן.

If they established an eiruv in the courtyards and also merged the courtyards of the alleyway, and one of the residents of the courtyard forgot and did not contribute to the eiruv in his courtyard, but did participate in the merging of the courtyards in the alleyway, they are permitted both here and there, as the merging of courtyards in the alleyway serves as an effective eiruv for the courtyards as well.

מבני מבוי ולא נשתתף ⁠— מותרין בחצירות ואסורין במבוי, שהמבוי לחצירות כחצר לבתים.

However, if one of the residents of the alleyway forgot and did not participate in the merging of courtyards that open into the alleyway, they are permitted to carry in the courtyards and prohibited from carrying in the alleyway, as the principle is: An alleyway is to its courtyards as a courtyard is to its houses.

גמ׳ מני? רבי מאיר היא, דאמר: בעינן עירוב ובעינן שיתוף.

Talmud: The Talmud asks: In accordance with whose opinion is this Mishnah? The Talmud answers: It is in accordance with the opinion of R' Meir, who said: We require an eiruv and we also require a merging of the courtyards in an alleyway, and one is not sufficient without the other.

אימא מציעתא: ״ואם נשתתפו במבוי מותרין כאן וכאן״, אתאן לרבנן דאמרי בחדא סגיא!

The Talmud asks: If so, say the middle clause of the Mishnah: And if they merged the courtyards in the alleyway, they are permitted to carry both here and there. We have arrived at the opinion of the Rabbis, who say that one is enough, and one does not need both an eiruv and a merging of alleyways.