Chapter 1, Mishna 1
To help us understand the laws of a breach in the side of a 3-sided alleyway, we look at the laws of an L-shaped alley. Rav says it is like two 2-sided alleyways coming together at the corner, Shmuel says it’s like a 3-sided alley with an opening in the side. 2-sided alleyways require more adjustment for Shabbos-carrying than three sided alleys because they connect one public thoroughfare to another and thus get more traffic. Need form of a doorway on one end and side post or crossbeam at the other per Rav. Adjusting an actual public thoroughfare for Shabbos-carrying requires even more. Actual door on one end that can be closed. There was one u-shaped alley in Nehardea where they adopted stringencies of both Rav and Shmuel. A baraisa teaches that we may neither adopt the leniencies of two sages who dispute each other nor their stringencies. Rather, we take on both the stringencies and leniencies of one sage or the other. We learn this from the disputes of Beis Shammai and Beis Hillel. But doesn’t the law always follow Beis Hillel? How can one choose to follow Beis Shammai?