735. Alleyways

Shabbos 16:24

The branches of a tree that droop downward so that they reach within three handbreadths of the ground (about 9”) form a valid enclosure. One should put things like straw and stubble between the branches and leaves, and tie them to the ground so that they will remain in place and not sway in an ordinary wind. If this has been done, it is permissible to carry under the entirety of the tree. This all assumes that the area beneath the tree is less than the space needed to plant two seah of seed. If the area is larger than that, one may only carry within four cubits because the space under the tree was enclosed for reasons other than to serve as a dwelling.

Shabbos 17:1                                                  

An alley with three walls is what’s called a “closed alley,” as opposed to an alley with only two walls, which is called an “open alley.” This includes an alley with two walls that are opposite one another so that pedestrians enter at one end and exit at the other.