Eruvin 10B

עושה פס גבוה עשרה במשך ארבע אמות, ומעמידו לארכו של מבוי.

One prepares a board 10 handbreadths high with a length of 4 cubits and stands it lengthwise down the middle of the alleyway, and thereby forms two small alleyways at the entrance to the alleyway, neither of which is more than 10 cubits wide.

אי נמי כדרב יהודה, דאמר רב יהודה: מבוי שהוא רחב חמש עשרה אמה ⁠— מרחיק שתי אמות ועושה פס שלש אמות.

Alternatively, one can act in accordance with the opinion of Rav Yehuda, as Rav Yehuda said: If an alleyway is 15 cubits wide, how does one reduce its width? He distances himself two cubits from one of the walls of the alleyway and prepares a board 3 cubits wide, thereby leaving an opening of only 10 cubits.

ואמאי? יעשה פס אמה ומחצה, וירחיק שתי אמות, ויעשה פס אמה ומחצה. שמע מינה: עומד מרובה על הפרוץ משתי רוחות לא הוי עומד!

The Talmud asks: And why must one reduce the width in this manner? One could also prepare a board 1½ cubits wide, and distance himself two cubits, and then prepare another board 1½ cubits wide, leaving the alleyway with an opening of only 10 cubits. Apparently, one may conclude from the fact that Rav Yehuda did not suggest this possibility that if the standing segment of a wall is greater than the breached segment only when one combines the standing segments from two directions, i.e., both sides of the breach, it is not considered as though the standing segment were greater.

לעולם אימא לך הוי עומד, ושאני הכא, דאתי אוירא דהאי גיסא ואוירא דהאי גיסא ומבטל ליה.

The Talmud rejects this: Actually, I would say to you that ordinarily it is considered as standing even when one must combine the standing segments on the two sides of the breach. However, it is different here, as the air, i.e., the two cubit opening, of this one side of the far board and the air, i.e., the 10 cubit opening, of this other side of the board come together and negate it. Therefore, in this case, the board that is farther from the wall cannot serve to close off the alleyway.