There are 39 categories of activity prohibited on Shabbat. One such category forbids moving an object from one domain to another.
According to Torah law, this encompasses three actions:
- Moving an object from an enclosed area (such as a private home, public building, or fenced-in area) to a major thoroughfare,
- Moving an object from a major thoroughfare to an enclosed area, or
- Moving an object more than six feet within a major thoroughfare.
The rabbis expanded the ban to any area that was not fenced or walled in.
An additional rabbinic prohibition forbids carrying in any area that is shared by the occupants of more than one dwelling. By setting aside a food item – generally, bread or matzah – that is shared by all dwellers within a boundary, all the dwellers are considered as if they are living in a common dwelling, thus exempting them from this added prohibition.
A kosher eruv, therefore, consists of:
1. A boundary / symbolic fence, as well as 2. A shared food item.
The boundaries of an eruv must be checked regularly. If the boundary is complete and contiguous in every element (i.e. not one of the elements of the boundary is missing or broken), a valid eruv can exist, and carrying within that eruv on Shabbat is permitted.