Masechet Eiruvin begins with the topic of tikkun mavoi – fixing a mavoi. A mavoi is a small private alleyway into which private courtyards (chatzer) open, with itself open to the public domain. Initially the residents cannot carry within the mavoi until the mavoi is fixed by placing a cross-beam (korah) or side post (lechi) at its entrance8. One must ask the following questions: What is the status of the mavoi9 prior to being fixed? What purpose does the korah or lechi serve?
R’ Yehonatan explains that a mavoi, on a biblical level, is considered a private domain. The Chachamim recognized that a mavoi is visibly open to the public domain. Consequently they were concerned that people would become confused and not differentiate between a mavoi and the public domain and mistakenly permit carrying in the public domain or mistakenly carry from the mavoi to public domain (both of which being biblically prohibited). The korah at the mavoi’s entrance therefore serves as a visual reminder for people to differentiate between the mavoi and public domain.
According to this explanation we understand the first Mishnah. The Chachamim require the korah be placed no higher the twenty amot. The reason being common with the limitation on the placement of the Chanukah candles - people don’t generally look at objects at such heights. As the korah is meant to serve as a reminder, placing it so high would be ineffective.
How would we then explain the position of R’ Yehuda that permits the korah to be placed above twenty amot? R’ Yehonatan explains that according to R’ Yehuda the korah does not function as a reminder, but rather as physical wall (pi tikrah ored u’sotem). One could explain that once the korah is placed at any height, the area is no longer a three-wall mavoi but rather a four-walled domain, lying outside the criteria of the rabbinic decree.
Rav Meir Pagrow explains that some Rishonim understand that even R’ Yehuda maintains the korah serves as a reminder. The difference is R’ Yehuda does indeed agree that people do not gaze at objects higher than twenty amot, nevertheless people do glance at objects placed higher then twenty amot. This is supported by the Gemara that explains that if the korah has an elaborate design then it can even be higher than twenty amot implying that people are not completely blind to object at those heights. Therefore while we require Chanukah candles be placed where people will gaze at it, R’ Yehudah maintains that for the korah to be a sufficient reminder it is enough that people glance at it.
The Rambam (Shabbat 17: 9) has a different understanding of mavoi:
A mavoi that has been fixed with a korah, even though one is allowed to carry within it like a private domain, if someone throws from it to the public domain… he is exempt since the korah serves as a reminder. However if the mavoi is fixed with a lechi and someone throws from it to the public domain he would have transgressed [a biblical prohibition], since the lechi serves as fourth wall.
Two important points come out of the Rambam. Firstly the korah and the lechi serve two separate purposes – the korah acts as a reminder while the lechi functions as a fourth wall. Secondly, it appears that a mavoi without fixtures is defined as a karmalit (neither a public nor private domain) – a region in which carrying is rabbinically prohibited. Consequently the lechi is required to convert the mavoi into a private domain. The chiddush (novel idea) of the korah is that its presence does not convert it into a private domain. Nevertheless it represents a karmalit in which one is allowed to carry.
8 One should note that the residents cannot carry from their own chatzer to the mavoi without first performing a shituf mavoi. However, before they can perform a shituf mavoi, the mavoi needs to be “fixed” with either a lechi or korah.
9 Please note: the term mavoi in this article refers to a mavoi satum, a mavoi with one end open to the public domain and the other end closed (as shown in the picture). A mavoi mefulash, a mavoi with both ends open to the public domain must be treated separately.
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