739. Is an Alley a Private Domain?

Shabbos 17:8

If the length of a courtyard exceeds its width, it is considered an alley and one may only carry in it after erecting a pole or a beam. If houses and courtyards do not open into an alley, or if only one house or one courtyard opens into it, or if an alley is not four cubits long, one may only carry in it after erecting two poles or a board four handbreadths wide (about a foot).

Shabbos 17:9

If an alley is not three handbreadths wide (about 9”), one may carry throughout its entirety without erecting a pole or a beam. This is because a gap of less than three handbreadths wide is considered closed up. If a horizontal beam is erected over an alley to enable people to carry in it like they do in a private domain, that doesn’t make it a true private domain. Therefore, one who throws something from such an alley to the public domain or vice versa is not liable. The purpose of the beam is to distinguish the alley. However, when a vertical pole is erected, the alley truly becomes a private domain. Therefore, one who throws something from the alley to the public domain or vice versa is liable. This is because the pole is considered a wall on the fourth side.