Chapter 1, Mishna 1
We make carrying possible on Shabbos in a three-sided alley (into which private courtyards open) by adding a crossbeam above the alley, where it opens into the public thoroughfare. The mishna said this crossbeam must be less than twenty cubits high – two reasons given: one Scriptural and one behavioral so we can fulfill our Scriptural obligations. What if the beam is an elaborate cornice? How does this connect to the 20 cubit limit on the roofing of a sukkah? What if the crossbeam is half above and hlaf below the legal limit? Why are the Sages so punctilious about legal limits and definitions? Is there a spiritual component to these limits? Construction of a sukkah is generally overseen by one person, the crossbeam of an alley is the responsibility of all the householders that use it. Which is more likely to be handled properly over time? “A pot owned by partners is neither hot nor cold!” Cubits are not standardized and neither are handbreadths, which can be “laughing” or “sad.” This leads to unintended consequences across different areas of law.