Eiruvin, chapter 1, part 1

Between yesterday and today, I utilized my time on lab supervisory duty to review Mishnayos Eiruvin. BEH, I plan to make a siyum on Seder Mo'eid on Yom Kippur, and have felt guilty that I haven't had a chance to do any chazara on the material that I've covered over the last few months (with the exception of a small chunk of Shabbos), so I'm glad that I at least saw the hardest maseches of the seder a second time. IYH, next week I'll try to do the same for Pesachim and Shekalim (which, of course, will entail finishing Shekalim the first time over Shabbos).

In the interest of doing another chazara on Mishanyos Eiruvin, I have decided to embark on a summary of the maseches:

Chapter 1 (Mavoi) discusses the halachic entity that is most often incorrectly referred to as an eiruv (this latter is not discussed until later in the maseches). The case that the mishnah uses is one of a mavoi, an area of a street that contains walls on two (or more) sides of it. In order to permit carrying within this area mid'rabbanan, the missing wall(s) have to be filled in. This can be accomplished without impeding the passage of traffic by use of a "korah", a "lechi", or a "tzuras ha-pethach". A korah is a crossbar of a given width and structural soundness that spans between the ends of two walls. A lechi is a fragment of a wall (10 tefachim high) that juts out of one of the walls' ends. My recollection is that we have some objection to using a lechi for our community 'eiruvin', but I don't recall what it is. In practice, the majority of community 'eiruvin' utilize the third non-invasive method of encompassing, a tzurath ha-pethach, or a "form of an entrance", which consists of any connection (even a wire or rope) between two poles, which is the analog to a lintel connecting two doorposts. The basis for this leniency is that any structure that has four walls will (usually) also have a door to allow entrance. This door does not preclude the structure from being considered a private domain, even when it is open, and thereby not completing the circuit of the walls; hence, even if the door were removed, it would be acceptable. A tzurath ha-pethach is considered the equivalent of a doorframe that has no door.

Now that we have established that a tzurath ha-pethach is equivalent to a solid wall, we can envision a circuit consisting entirely of tzuroth ha-pethach - hence, the traditional community "eiruv" consisting mostly of phone or electric wires, with koroth added as needed.