As we learned in chapter 21, one may transport items on Shabbat within a private domain (reshut ha-yaĥid) but not more than four amot within a public domain (reshut ha-rabim) and not from a public to a private domain and vice versa.
An eruv, however, transforms a reshut ha-rabim into a reshut ha-yaĥid, thus allowing people to transport items within that area without limit, as well as to transport items from homes and yards into that area, and vice versa.
An eruv has two components, one pertaining to the area that will be transformed into a private domain, and the other relating to the people who will live within the eruv’s boundaries. In order to define the area as a single private domain, it must be enclosed by a fence. However, this is not enough. The people who live within the area must also establish a partnership with one another. This is done by means of two meals worth of food that is owned collectively by everyone and from which anyone may partake. This communal food is called an eruv (literally “merging”) because it merges or joins together all the homes and yards, turning them into a single private domain. Nevertheless, it has become commonplace to call the enclosure itself an eruv as well, and we follow this convention.
As we learned (21:2-3), there is a difference between a reshut ha-rabim by Torah law and a semipublic domain (karmelit), which is treated as a reshut ha-rabim by rabbinic law. In order to permit carrying in a reshut ha-rabim by Torah law, the area must be enclosed by a wall or fence at least ten tefaĥim high (approximately one meter), and the gates that people use to enter the enclosure must be locked at night (SA 364:2).
However, a karmelit, considered a public domain on the rabbinic level, need not be enclosed by an actual fence. Rather, it is sufficient to surround it with tzurot ha-petaĥ (defined in the following section) to transform it into a reshut ha-yaĥid (SA 362:10-11).