פירוש על עירובין 7:4

נתן עליו נסר – from one rim/border [of the trench] to the other, like a kind of bridge, which is like an opening.

Introduction This mishnah continues to deal with a ditch separating two courtyards.

שתי גזוזטראות – as a kind of protruding ceiling/roofing from the private domain on the public domain, and he places a plank that is four [handbreadths] wide from one side to the other, and it is also an opening.

If he placed over it a board four handbreadths wide, and so also where two balconies are opposite one another, the residents may prepare two eruvin or, if they prefer, only one. [If the board] was less wide two eruvs may be prepared, but not one. If someone “bridges” the ditch with a board four handbreadths wide, the two courtyards can now be treated as one. The board is to the ditch like the opening was to the wall both serve as passages from one courtyard to the other. The second clause of the mishnah states the opposite if the board was less than four handbreadths wide, then it does not serve as a passage because people would be hesitant about crossing. In such a case each courtyard must set up its own eruv. The mention of the two balconies is abbreviated here and must be fleshed out. The two balconies can join two courtyards if there is a four handbreadth wide board which bridges the two. In such a case they may make either two separate eruvin or one for both. If there is no such board, then they must make separate eruvin.

פחות מכאן – that if the plank is not four [handbreadths] wide, a person will fear to pass upon it and its use won’t be pleasant [and hence requiring two Eruvin – one for each side].