Chapter 4, Mishna 4, 5
Three multi-home courtyards in a row can establish a joint eruv by depositing food in a basket in one of the homes in the middle courtyard. If the outer courtyards make separate arrangements with the inner courtyard, however, AND they split the two food deposits into separate baskets, no one can carry anywhere on Shabbos. Why? One who is heading home as Shabbos is about to start, and he knows 1) his house is 4000 cubits aways and 2) there’s a particular tree 2000 cubits away in the road home, can declare his Shabbos dwelling “is beside the trunk of the tree,” and he’ll be able to walk home on Shabbos. But if he says his home is “under the tree” he won’t. Why?