933. A Wall Between Courtyards

Eiruvin 3:15

Let’s say that there is a wall four handbreadths wide (about a foot) in-between two courtyards, ten handbreadths tall (about 30”) to one courtyard and ground level to the other. The width of the wall is considered an extension of the courtyard for which it is at ground level since it is easy for the residents of that courtyard to access it and difficult for the residents of the other courtyard. Similarly, if there is a trench in-between two courtyards, ten handbreadths deep for one of them and ground level for the other, the width of the trench is given to the courtyard for which it is at ground level because those residents can access it more easily.

Eiruvin 3:16

Let’s say that there’s a wall between two courtyards that is lower than the higher courtyard and higher than the lower courtyard so that the residents of the higher courtyard generally use of the wall’s surface by lowering things onto it, while the residents of the lower courtyard use the wall’s surface by throwing things onto it. In such a case, the residents of neither courtyard are permitted to use the wall on Shabbos unless they make a single eiruv between them. If they do not do this, it is prohibited to move objects that were left on the wall into the house, nor to bring objects from the houses to the surface of the wall.