An eruv (; , , also transliterated as eiruv or erub, plural: eruvin or eruvim) is a ritual halakhic enclosure made for the purpose of allowing activities which are normally prohibited on Shabbat (due to the prohibition of hotzaah mereshut lereshut), specifically: carrying objects from a private domain to a semi-public domain (carmelit), and transporting objects four cubits or more within a semi-public domain. The enclosure is found within some Jewish communities, especially Orthodox ones.
An eruv accomplishes this by symbolically integrating a number of private properties and spaces such as streets and sidewalks into one larger "private domain" by surrounding it with mechitzas, thereby avoiding restrictions of transferring between domains. Often a group constructing an eruv obtains a lease to the required land from a local government.
An eruv allows Jews to carry, among other things, house keys, tissues, medication, or babies with them, and to use strollers and canes – all of which are otherwise forbidden from being carried outside one’s home during Shabbat, in the absence of an eruv.
Definition
The prohibition of transferring between domains
In Jewish tradition it is commonly said that "carrying" is forbidden on Shabbat. Specifically, "transferring between domains" () is considered one of the 39 categories of activity prohibited on Shabbat.
The halacha of Shabbat divides spaces into four categories: Private domain (reshut hayachid), such as a house Public domain (reshut harabim), such as a very busy road Semi-public domain (karmelit), which includes most other places Neutral domain (makom patur), such as the flat space on top of a pole
A domain is defined as public or private based on its degree of enclosure, not its ownership. The rules here are complex, and expertise is needed to apply them.
On Shabbat, it is forbidden to transfer an object from one domain to another, including from one person's house to another adjacent house. The only exception is transferring to or from a neutral domain (which is rarely relevant).
In addition, it is also forbidden to transfer an object for a distance of 4 cubits (approximately 2 metres; 7 feet) within a public domain or karmelit ().
While biblical law prohibits carrying objects between private and fully public domains on Shabbat, rabbinic law extends this restriction to carrying between a private domain and a semi-public karmelit as a safeguard of the biblical law. The rabbinic prohibition of carrying between a private domain and a karmelit is relaxed whenever an eruv is in place.
Eruv chatzerot
The term eruv is a shortening of eruv chatzerot (), literally a "merger of [different] domains" (into a single domain). This makes carrying within the area enclosed by the eruv no different from carrying within a single private domain (such as a house owned by an individual), which is permitted.
The eruv typically includes numerous private homes, as well as a semi-public courtyard whose ownership is shared by them. To enact the merger of the homes and courtyard into a single domain, all home owners as well as owners of the courtyard must pool together certain foodstuffs, which grants the area of the eruv the status of a single private domain. As a precondition for this merger, the area must be surrounded by a wall or fence.
In many cases (for example, within an apartment complex or walled city) the demarcation of the shared area consists of real walls or fences. Building walls may also be used, and in some cases so may a natural wall such as a river bank or steep hill.
Walls may include doors and windows. As such, the wall may even consist of a series of "doorframes" with almost no wall between them. Poles in the ground form the "doorposts" of the doorframe, and rope or wire between the poles forms the "lintel" of the doorframe. In modern cities, it is typical for the majority of an eruv to consist of such doorframes, using utility poles and wires.
When a "doorframe" is used as part of an eruv, it is required that the "lintel" rest on top of the "doorpost", rather than being attached to the side of the "doorpost". Since the "lintel" is frequently a utility wire which runs along the side of the utility pole, the pole cannot be used as "doorpost". In this case, an additional "doorpost", known in Hebrew as a lechi (pl. lechai'in), is attached to the side of the utility pole. This typically takes the form of a thin plastic pipe attached to the side of the utility pole, and the wire runs directly overhead of this pipe.