However this one focuses on a special issue: the eruv - a device which allows public space to be considered “private” for the purpose of Shabbat laws.
Remember that in a private space there is far more permissible on Shabbat than in public ones. The moving of objects, for example.
Our Mishnah begins with a discussion of a blind alley. Presumably bounded on three sides by private courtyards but opening onto public space, it is defined as an enclosed space with the presence of a cross beam over the entrance. However:
[A CROSS-BEAM SPANNING] THE ENTRANCE [TO A BLIND ALLEY] AT A HEIGHT OF MORE THAN TWENTY CUBITS SHOULD BE LOWERED. R. JUDAH RULED: THIS IS UNNECESSARY.
There is extensive discussion of where these numbers come from. Is it from the measurements of the entrance to the Tabernacle, the Sanctuary? Or perhaps it comes from the limits on the size of a sukkah?
Elsewhere we have learnt: A sukkah which [in its interior] is more than twenty cubits high is unfit, but R. Judah regards it as fit.
This seems to fit! But what is strange is that the Mishnah here suggests a fix (lower the beam!) That is not the case for the sukkah. Why not?
[In respect of a] sukkah, since it is a Torah ordinance, it [was proper categorically to] rule it, ‘unfit’; in respect of the ENTRANCE, however, since [the prohibition against moving objects about in the alley is only] Rabbinical, a remedy could well be indicated.
Thus the distinction between d’rabbanan and d’orita. There is more flexibility in the laws imposed by the rabbis through interpretation than those directly written.