By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
The melachah ([one of 39 types of prohibited] work) of hotza’ah(carrying) on Shabbos includes carrying mereshus l’reshus (from one [type of halachic] domain to another) or from one place to another within reshus harabim (the public domain [within or near a major thoroughfare]). In a reshus hayachid (private domain) we may carry unrestricted.
However, Shlomo Hamelech (the king, 10th century BCE) instituted the concept of an eruv chatzeiros (mixed [ownership of] courtyards) for carrying from one reshus hayachid to another or in a common area between neighbors. While the common area is fenced in, and therefore a bona fide reshus hayachid, it can be confused with a reshus harabim, and would therefore be subject to restriction. Through the takanah (institution) of an eruv,* where the neighbors band together by sharing food, they essentially create one inclusive domain out of their individual residences.
There are many intricate laws regarding the parameters of such an eruv between neighbors in a single building of separate apartments, those that share a common courtyard and private houses that border each other. In a very general summary, these are the three requirements for establishing an eruv chatzeros:
Fenced In: Mechitzos (partitions)—physical demarcations of at least ten tefachim (hand-breadths, a little less than a meter in total) high around the eruv zone must be present. In areas where there is a pirtzah (breach) in a wall or fence, it is sometimes possible to bridge the gap with a tzuras hapesach (a doorframe) constructed in accordance with specific criteria.
Mixed Crowd: If the eruv zone includes a non-Jewish resident among two or more Jewish residents, sechiras reshus (rental of territory) from the non-Jewish residents must be arranged.
Honestly Open: Less known than the other criteria, there is a rule of accessibility on the shared properties of an eruv zone. If there is a barrier of ten tefachim or higher between courtyards to be included, a door or a window of at least four by four tefachim (six feet by six feet) must allow access from one property to the next in order for the eruv to be effective.
*The enclosures of tzuras hapesach—consisting of strings or cables hung between posts or poles—that are erected in urban areas categorized as karmelis (“semi-private” domain, i.e., a Rabbinically restricted area that is similar to reshus harabim) are more accurately termed mechitzos, although they are colloquially called “eruv” too. The food-shares in urban areas, kept in a central location on behalf of all city residents, are usually referred to as shitufei mevo’os (associations of pathways) in contrast to eruvei chatzeros—food shares for courtyards. Shitufei mevo’os were also instituted by Shlomo Hamelech and have similarities to eruvei chatzeros, but their complex rules and parameters, as well as those of tzuras hapesach, are beyond the scope of this halachah. (See Halachah #662 about restrictions outside an eruv in both a karmelis and a reshus harabim and some of the workarounds in an emergency.)