Eiruvin 7:2-3

Eiruvin 7:2

If the wall between courtyards is ten handbreadths tall (about 30”) and four handbreadths thick (about 12”), the courtyards may only make two eiruvs, not one. If there was fruit on top of the wall, residents on each side may go up and eat fruit there but they may not take it down into their courtyards. If the wall is breached but the gap is smaller than ten cubits (about 15’), they may make two separate eiruvs or one joint eiruv since the breach serves as a door. If the breach is any larger, they may only make one eiruv, not two. (Since they are breached wider than a doorway, they are functionally one big courtyard.)

Eiruvin 7:3

If a trench between two courtyards is ten handbreadths deep and four handbreadths wide, they may only make two eiruvs, not one. This is so even if the trench has been filled with straw or stubble (which typically remain there only temporarily, to serve as a source of cattle fodder). If the trench has been filled with dirt or gravel, they may only make one joint eiruv, not two (since the courtyards are effectively combined).