According to the Gemara, aside from the cross beam that has been discussed, carrying in a Mavoy (see daf 2) is also permitted if a “Lehi” (side post) is placed vertically against one of the walls at the entrance to the Mavoy. A Lehi is a pole, plank, or other object that is at least ten tefahim high. Like the cross beam, it serves as a fourth wall and/or as a “heker” (reminder) to indicate the beginning of the public domain so that people will not transfer objects from the Mavoy to the adjacent Reshut ha’Rabim (= public domain).
The Gemara quotes Rami bar Hama in the name of Rav Huna as saying that in the event that the Lehi is part of the structure of the Mavoy (i.e. it was not placed there specifically for the purpose of being a Lehi), if it protrudes from the wall into the opening of the Mavoy less than four amot (=cubits), it can function as a working Lehi. If, however, it is longer than four amot, then it will not work, and a different side post is needed to permit carrying in the Mavoy. The reason for this, according to Rashi, is that if the Lehi is longer than four amot – and it was not erected to act as a symbolic Lehi – it is merely part of the Mavoy’s wall, but it is not long enough to be considered a wall to close the Mavoy off from the public domain properly.
Rav Huna the son of Rav Yehoshua comments that this is only true if the Mavoy’s entrance is eight amot wide or greater; if it is seven amot, then even if the four ama wall cannot act as a Lehi, nevertheless carrying will be permitted because that side post sufficiently seals off the entrance to the alleyway. Rav Ashi argues that even if the entrance is exactly eight amot, a protruding side post of four amot will permit carrying in the Mavoy for one of a number of reasons:
If the wall is longer than the open space, it is permitted because the majority of the entrance is closed up.
If the wall is shorter than the open space, it is permitted because it can be considered a functioning, symbolic Lehi
If they are exactly the same size, then it falls into the category of an uncertainty with regard to rabbinic law – a doubtful circumstance in a rabbinic situation, where we are lenient.
The suggestion that the standing wall may be precisely the same size as the open area is connected with a general dilemma that is often discussed in the Gemara – is it ever possible for us to establish that two things are precisely identical? The Rishonim (Tosafot and others) debate this issue, and some conclude that Rav Ashi’s argument indicates that we, as fallible human beings, can never conclude with certainty that two things are precisely equal.