Eruvin 85B

והאמצעי ⁠— אסור.

and the middle ruin is prohibited to both of them.

יתיב רב ברונא וקאמר להא שמעתא. אמר ליה רבי אליעזר בר בי רב: אמר רב הכי? אמר ליה: אין. אחוי לי אושפיזיה. אחוי ליה. אתא לקמיה דרב, אמר ליה: אמר מר הכי? אמר ליה: אין.

Rav Beruna sat and stated this halakha in the name of Rav. R' Elazar, a student of the Torah academy, said to him: Did Rav actually say this? Rav Beruna said to him: Yes, he did. He said to him: Show me his place of lodging, and I will go and ask him myself. He showed him where Rav lived. R' Elazar came before Rav and said to him: Did the Master actually say this? He said to him: Yes, I did.

אמר ליה, והא מר הוא דאמר: לזה בשלשול ולזה בזריקה ⁠— שניהן אסורין!

R' Elazar then said to Rav: Since you prohibit using the middle ruin, you evidently maintain that one person renders it prohibited for another by way of the air. That being the case, it must be that you permit the resident of each house to use the adjacent ruin because one’s use of the ruin, while not convenient for him, is more convenient than the other person’s usage. But wasn’t it the Master himself who said: With regard to a place that can be used by the residents of the one courtyard only by lowering an object down to it and by the residents of another courtyard only by throwing an object on top of it, so that neither courtyard has convenient access to it, both sets of residents are prohibited from using it, although lowering an object is more convenient than throwing it?

אמר ליה: מי סברת דקיימי כשורה? לא ⁠— דקיימי כחצובה.

Rav said to him: Do you think that we are dealing with a case of 3 ruins positioned alongside each other in a straight line? No. They are arranged in the form of a tripod, i.e. in a triangular form. In other words, two of the ruins, each adjacent to one of the houses, are located next to each other; the third is positioned adjacent to one side of the other two, near both houses. The middle ruin is prohibited to the residents of both houses because both houses have equally inconvenient but direct access to it. However, each of the other ruins is permitted to the resident of the adjacent house, as he has direct access to it, while the resident of the other house can reach it only through the air of the ruin nearest to him, and Rav maintains that one person does not render it prohibited for use by another by way of the air.