Considering a stone to be hollowed out, such that it becomes like a corner enclosure for a well: Eruvin 19b
Considering a tree or wall as an enclosure: Eruvin 15a
When are "Peshutin," straight pieces of wall, needed in the space between the corner-walls: Eruvin 19b
Waiving the principle [Parutz Merubeh Al haOmeid] that a gap which is larger than its surrounding walls cancels out the walls themselves: Eruvin 10a, 11a
Using a natural growth as a wall: Eruvin 15a
Counting a growth of reeds which form a corner, as one of the corner enclosures [Deyumdin]: Eruvin 15a, 16b, 19b
Miscellania
Absence of these well-enclosures outside Israel, because of the lack of Torah study centers to which people could travel: Eruvin 21a
Absence of these well-enclosures in Babylon, since there was plenty of water available there, and so this leniency was unncessary: Eruvin 21a
