Some thoughts on learning Eruvin. Plus: What happens when a backyard is owned by more than one person... Or when the alleyway runs between sites that are nobody's property, but also not a public domain. For example, the sea. For example, a garbage heap. Even R. Yehudah HaNasi didn't have an easy answer -- which leads into the question of how the alleyway itself was going to be treated, and whether it will be set off by a partition or just demarcation. Also: A centipede alleyway. And: An alleyway with different lengths of bordering walls, so should the koreh (crossbeam) be placed diagonally across the alleyway, from the shorter one to the longer one...? In doing so, the conspicuous marker that identifies the transformed space as being rendered a private domain. But that's a matter of dispute. Until the case of a diagonal that is longer than 10 amot... Also, can you use the space under the koreh? Is it whether the koreh is a demarcation? Or an actual partition? The daf traces these positions to their logical implications. Note too that the rabbinic nature of this question (by definition) of partition vs. recognition, or an identification of both together.