934. Areas That Are a Single Private Domain

Eiruvin 3:17

Let’s say that there’s a ruin, which is a private domain, in-between two houses. If the residents of each house can use the ruin by throwing things into it, then each of them impedes the other’s ability to use the ruin in the absence of an eiruv. If it’s easy for residents of one house to use the ruin and more difficult for the others because of a difference in elevation between the ruin and their home, then the right of access is given to those with the easier access, who may use the ruin on Shabbos by throwing things into it.

Eiruvin 3:18

The following are all considered a single private domain: all the roofs in a city even though some are higher than others; all the courtyards; all enclosures that were enclosed for purposes other than dwelling and that are smaller in area than needed to plant two seah of seed (about 150 feet by 75 feet); the tops of all the walls; and all alleys that have had either a vertical post or a horizontal beam installed. If an item was left in one of these places when Shabbos began, it may be transported to another of these places, though one may not transfer between houses and these areas without an eiruv.