Shabbos, Chapter 11, Mishnah 2. According to some versions the Mishnah opens with the word "keitzad" - "how so?", and according to that version it is a continuation of the previous Mishnah, where Rabbi Akiva and the Sages disagreed in a case where a public domain separates two private domains: whether one who carries an object from one private domain to the other through the airspace of the public domain is liable.
A difficulty arises, however, from the fact that our Mishnah deals with carrying above ten tefachim, and as we mentioned in the previous Mishnah, even Rabbi Akiva agrees that above ten tefachim there is no liability. Therefore, according to the version of the Yerushalmi the word "keitzad" is not read here, and this is a new, independent case.
Two balconies opposite one another:
The Mishnah states: "Shtei gezuztra'os zo k'neged zo birshus harabim" - two balconies standing opposite one another over a public domain. In other words, a main thoroughfare runs down the middle, and on either side of it the balconies project, and they are private domains situated above the public domain.
In such a case, "hamoshit v'hazorek mizo lazo - patur" - whether one passes an object from one balcony to the other by hand or throws it, he is exempt, since this is a transfer above ten tefachim.
If both were on the same level:
The exception is when both balconies are on the same side of the public domain - "b'diyota achas", meaning on the same story and in the same building. In such a case the one who passes it by hand is liable and the one who throws it is exempt: one who passes an object from one balcony to the other is liable for carrying out, while one who throws it is exempt.
The source in the service of the Levites:
Why was this exceptional ruling established? Because "shekach haysah avodas haLeviim" - this is how the Levites conducted their service in the wilderness, when they carried the beams of the Mishkan and transferred them from wagon to wagon. Each wagon was a private domain, the public domain separated between them, and the wagons stood one behind the other along the road - which is just like two balconies on the same side of the street.
In the words of the Mishnah: "Shtei agalos zo achar zo birshus harabim - moshitin hakerashim mizo lazo, aval lo zorkin" - two wagons one behind the other in the public domain: they would pass the beams from one to the other, but they would not throw them. Since this transfer replicates what was done in the wilderness, it carries liability even though it is above ten tefachim; but the liability applies specifically in the manner that was done in the wilderness - by passing and not by throwing, for the Levites did not throw the beams to one another.
The rim of a pit and a rock:
The Mishnah continues: "Chulyos habor v'hasela shehen g'vohin asarah v'rochban arba'ah" - a rim surrounding a pit or an excavation, and likewise a rock, that are ten tefachim high and four tefachim wide, are a private domain.
"Hanotel mehen v'hanosen al gaban - chayav" - one who takes from atop them and places in the public domain, and likewise one who takes from the public domain and places on top of them, is liable, since these are a private domain.
"Pachos mikan - patur" - if they do not have these measurements, he is exempt, for this is not a private domain but a karmelis, and by Torah law it is permitted to carry from a public domain to a karmelis and back.
Why did the Mishnah mention the rim of a pit, and what relevance does the pit have here? Seemingly it should be enough that the rim itself is four tefachim wide and ten tefachim high. The Gemara explains, however, that the case is one where the rim on its own does not have ten tefachim, and it reaches ten only when combined with the depth of the cavity of the pit. The two combine together to render the rim a private domain, and therefore there is liability on Shabbos.
In summary: In this Mishnah we learned the law of two balconies: when they are opposite one another on the two sides of the public domain, both the one who passes and the one who throws are exempt, because the transfer is above ten tefachim; but when both are on the same level, the one who passes is liable and the one who throws is exempt, following the service of the Levites, who passed the beams from wagon to wagon and did not throw them. We also learned the law of the rim of a pit and a rock: those that are ten high and four wide are a private domain and carry liability, while less than that is exempt because it is a karmelis, and that the depth of the cavity of the pit combines with the height of the rim to complete it to ten tefachim.