Eiruvin 10:8-9

Eiruvin 10:8

If the branches of a tree come to within three handbreadths of the ground (about 9”), one may carry under the tree on Shabbos. If the tree’s roots are three handbreadths high, one may not sit on them on Shabbos (because we are not permitted to make use of a tree on Shabbos). The door propped into a storage shed, thorns used to fill a breach and mats may not be used to close openings on Shabbos (because, not being proper doors, using them resembles an act of building) unless the gap being filled is elevated from the ground (in which event their purpose is evident).

Eiruvin 10:9

Rabbi Meir said that one may not stand in a private domain and unlock a door in the public domain, or vice versa, unless he prepared a partition ten handbreadths high (about 30”). The Sages replied that a certain market where they fattened animals in Jerusalem used to lock up and place their key in a cubby above the door (so that they were putting it in a private domain even though they were standing in a private domain). Rabbi Yosi also recalled the incident, though he said that it had been a wool market. [The Gemara explains that since Jerusalem was a walled city whose gates were locked at night, its markets were actually a carmelis rather than a public domain.]