Eiruvin 7:4-5

Eiruvin 7:4

If there is a trench in between courtyards and someone covered it with a board four handbreadths wide (about a foot), they may make one joint eiruv or two separate eiruvs, as they prefer. If the board is narrower than this, they may only make two separate eiruvs. The same rule applies when two balconies face one another and one bridges the gap with a board of four handbreadths, or fewer.

Eiruvin 7:5

If a haystack that separates two courtyards is ten handbreadths tall (about 30”), they may only make two separate eiruvs, not one joint eiruv, and their cattle may eat from either side. If the stack is reduced to below ten handbreadths, they only make one joint eiruv, not two separate eiruvs.