(Mishnah): If he put [the Eruv] in a tree above 10 Tefachim, it is not an Eruv;

If it is below 10 Tefachim, it is an Eruv;

If it is in a pit, even 100 Amos deep, it is an Eruv.

If he put the Eruv on top of a reed or pole, if it was initially detached and later inserted into the ground, it is an Eruv.

If one put it in a cabinet and locked it, and lost the key, [even so] it is an Eruv;

R. Eliezer says, if he does not know that the key is in its place, it is not an Eruv.

(Gemara): [Below 10 Tefachim,] it is an Eruv, but one may not move it (because he uses a tree). If it is below three [Tefachim], it is permitted.

Question: If it is an Eruv, it should be permitted to move it. If it is forbidden to move it, it should not be an Eruv!

Answer: It is [considered] proper, for he could transgress Shevus (using a tree) and eat it.

Question: If so, even above 10 [it should be Kosher!

Answer (Rav Yehudah citing Shmuel): The case is, the trunk was four [by four Tefachim, so the tree is Reshus ha'Yachid; he is in Reshus ha'Rabim.]

(R. Mana): This is when Reshus ha'Rabim surrounds it on every side [so he cannot bring it down], and he said "my Shevisah is below it.'' (If his Shevisah were above, he could climb the tree and eat his Eruv.)