The Mishna (2a) introduces us to a disagreement between Rabbi Yehuda and the Hakhamim as to whether the cross beam – the board that symbolically closes the open end of a Mavoy – can be placed above 20 amot (= cubits) from the ground (Rabbi Yehuda) or needs to be 20 amot off the ground or lower (Hakhamim).
(Note: A Mavoy is an alleyway that is surrounded by courtyards on three sides, with the fourth side open to a Reshut ha’Rabim – the public domain. The people of the surrounding courtyards must pass through the Mavoy in order to go out to the street. Carrying in a Mavoy is rabbinically prohibited, because the Mavoy is similar to a public domain in that many households make use of it as a thoroughfare.)
The first suggestion of the Gemara is that Rabbi Yehuda and the Hakhamim agree that the cross beam is modeled after the entrance to the Temple. They differ as to whether it is similar to the doorway to the Sanctuary, which was 20 amot high, or the doorway of the Entrance Hall, which was 40 amot high. The Gemara concludes, however, that this disagreement does not have its source in a dispute about how to understand the passages in the Torah describing the entrance to the Temple; Rabbi Yehuda’s lenient position stems from his comparison to the entrances to the palaces of kings, which are usually very wide and very tall.
Taking a different approach, the Jerusalem Talmud (and similarly the Me’iri and the Bartenura) explains that the disagreement is based on a basic difference of opinion about the purpose and significance of the cross beam. According to the Hakhamim the purpose of the symbolic board is to act as a “heker” – a recognizable statement and reminder that the Mavoy ends and the public domain begins. If the board is placed above 20 amot, it may not be seen and thus does not fulfill that purpose. According to Rabbi Yehuda, the cross beam acts as a real wall – from a legal standpoint – based on the principle “Pi Tikra Yored V’Sotem” (we consider the edge of a wall to go down and close off the area), and this legal fiction works even at a height of more than 20 amot.