Eruvin 67B

אפילו פתוח לקרפף.

Even if it opens into an enclosure, this is considered its main entrance, rather than the one that opens into the alleyway.

רבה ורב יוסף דאמרי תרוייהו: גוי בית סאתים ⁠— אוסר, יותר מבית סאתים ⁠— אינו אוסר.

It is Rabba and Rav Yosef who both say: The halakha in such a case depends on the identity of the owner of the courtyard. With regard to a courtyard owned by a non-Jew, if the enclosure behind his courtyard is the size of two beit se’a or less, he renders it prohibited for the Jewish residents of the alleyway to carry. An enclosure of this size is not large enough for all the non-Jew’s needs, and therefore his main entrance is the one that opens into the alleyway. However, if the enclosure is greater than the size of two beit se’a, he does not render it prohibited for the residents of the alleyway to carry, as such an enclosure is sufficient for all his needs.

וישראל בית סאתים ⁠— אינו אוסר.

On the other hand, with regard to a courtyard owned by a Jew, if the enclosure is the size of two beit se’a or less, he does not render it prohibited for the other residents of the alleyway to carry, even if he did not join in an eiruv with them. Because he has the option of carrying in such an enclosure on Shabbat, he would not carry in the alleyway, as it is more convenient for him to carry in a place that belongs exclusively to him.

יותר מבית סאתים ⁠— אוסר.

However, if the enclosure is greater than the size of two beit se’a, in which case it is prohibited to carry there, the Jew would carry only by way of the alleyway. Therefore, he renders it prohibited for his fellow residents of the alleyway to carry unless he establishes an eiruv with them.