Eruvin 78A

זיז היוצא מן הכותל ארבעה על ארבעה, והניח עליו סולם כל שהוא ⁠— מיעטו.

If a projection 4 by 4 handbreadths in area extends from a wall, and one placed a ladder of any width against it, if the rungs of the ladder are less than 3 handbreadths apart, he has diminished the height of the wall by means of this ladder and projection.

ולא אמרן אלא דאותביה עליה, אבל אותביה בהדיה ארווחי ארוחיה.

The Talmud qualifies this statement: And we said this only in a case where one placed the ladder directly against the projection, so that the ladder serves as a passage to it. However, if he placed it adjacent to the projection, he has merely widened the projection, while the ladder remains separate from it. Consequently, the projection does not have any connection to the ground, and a projection that is not within 3 handbreadths of the ground does not diminish the height of a wall.

ואמר רב נחמן אמר רבה בר אבוה: כותל תשעה עשר ⁠— צריך זיז אחד להתירו.

And Rav Naḥman said that Rabba bar Avuh said: If a wall between two courtyards is 19 handbreadths high, it requires one projection to render the use of the wall permitted. If there is a single projection in the middle of the wall, with a ladder of any width resting against it, it is considered a passageway between the courtyards, as the projection is within 10 handbreadths of the top of the wall.

כותל עשרים ⁠— צריך שני זיזים להתירו. אמר רב חסדא: והוא שהעמידן זה שלא כנגד זה.

However, if the wall is 20 handbreadths high, it requires two projections to render the use of the wall permitted, one within 10 handbreadths of the ground, and the other within 10 handbreadths of the top of the wall. Rav Ḥisda said: And this applies only where he positioned the projections not directly one above the other, so that it is possible to use a ladder to climb from one projection to the other.