As we have seen (section 1 above), in order to transform a reshut ha-rabim or karmelit into a reshut ha-yaĥid in which carrying is permitted, it is not enough to enclose it in a fence or tzurat ha-petaĥ. A partnership must also be formed among all the residents in the enclosed area. This is accomplished by means of bread owned jointly by every resident of the enclosed area. It is not necessary to use bread made from one of the five species of cereal grains; rice bread is also acceptable (SA 366:8). If fewer than eighteen people live in the enclosed area, it is sufficient to have enough jointly-owned bread for each person to have the volume of one grogeret (dried fig). If there are exactly eighteen people, a volume of eighteen grogarot is required, the quantity considered the equivalent of two meals’ worth. For a group of more than eighteen residents – even a thousand – the amount of bread required to form a partnership between all of them does not change, but remains uniform at two meals’ worth.
The poskim disagree about the precise quantity of two meals’ worth of food. The accepted ruling is that le-khatĥila the volume of eight eggs (about 400 ml) should be used, while be-di’avad the volume of six eggs (about 300 ml) is sufficient (SA 368:3). As mentioned above, this communal food is called an eruv (literally “merging”) because through it everyone is merged or joined together, and their enclosed property is defined as a reshut ha-yaĥid.
Since the eruv bread belongs to all residents of the city, any of them may eat it whenever he wishes. If the eruv was eaten during Shabbat, the residents may continue to carry for all of that Shabbat. Since the eruv was there during bein ha-shmashot on Friday (when Shabbat began), the partnership was already formed. For the next Shabbat, however, a new eruv must be set aside.
On a kibbutz or anywhere else where all the residents eat together in a communal dining hall, it is sufficient to erect a tzurat ha-petaĥ around the area. It is not necessary to put aside two meals’ worth of food in such a case, since the food in the communal kitchen serves to form the general partnership necessary for an eruv.