1) HALACHAH: THE SIZE OF A "SHIYUR"
OPINIONS: The Mishnah (59a) teaches that an ordinary Eruv Chatzeros cannot be made for an Ir Shel Rabim. An Ir Shel Rabim ("a city of the public") is a city that has some characteristics of a Reshus ha'Rabim (see Insights to Shabbos 6:2-3). According to RASHI, although the city has thoroughfares through which 600,000 people pass, it does not have streets that are 16 Amos wide, and thus it is not a Reshus ha'Rabim mid'Oraisa (since it does not fulfill the two conditions of 600,000 people and 16 Amah-wide streets; see Background to the Daf for other definitions of an Ir Shel Rabim).
The Rabanan decreed that a single Eruv Chatzeros does not suffice for an entire Ir Shel Rabim. The decree was enacted lest people forget that an Eruv Chatzeros was made in this city, and that the city is not a Reshus ha'Rabim mid'Oraisa. They might mistakenly assume that they may carry in a Reshus ha'Rabim mid'Oraisa.
The only way to make an Eruv Chatzeros for an entire city is to leave out one section of the city from the Eruv (a "Shiyur"). The residents of this section may then make a separate Eruv Chatzeros for themselves, but they may not carry into the main part of the city (and vice-versa). The Shiyur causes people to remember that an Eruv was made in the city for the purpose of carrying.
Rebbi Yehudah and Rebbi Shimon argue about the number of houses that must be left out of an Eruv in an Ir Shel Rabim. Rebbi Yehudah says that 50 residents must be left out. Rebbi Shimon says that three courtyards with two houses each must be left out. The Gemara (60a) records another opinion: Rebbi Yitzchak says that it suffices to leave out a single courtyard occupied by a single house.
What is the Halachah in this case? Moreover, why is a Shiyur not left out when we make an Eruv for a city nowadays?
(a) TOSFOS (DH Gemara) rules in accordance with Rebbi Shimon, because Rav Chama bar Gurya says in the name of Rav that the Halachah follows his view. Furthermore, it is apparent from Abaye's question that Rebbi Yitzchak, who is only an Amora, cannot argue with Rebbi Shimon's ruling. This explains why Abaye wanted to know whether Rebbi Yitzchak's statement was based on a tradition or on logic. If it was based on tradition, then it is a Halachic ruling representing an unrecorded opinion of the Tana'im in this matter which should be recorded (even if it is not the Halachic opinion). If it was based on logic, then Rebbi Yitzchak is simply saying, "I would have thought that one house can also be a Shiyur, but since the Tana'im rule otherwise, my opinion is inconsequential."
(b) However, the RIF, RAMBAM, ROSH and other Rishonim rule in accordance with Rebbi Yitzchak, that one courtyard with one house suffices. They base their ruling on the principle that we always follow the more lenient ruling in the laws of Eruvin. Moreover, we see from Abaye's testimony earlier that a grain silo sufficed as a Shiyur in Pumbedisa, which implies that Abaye ruled like Rebbi Yitzchak. (Tosfos (DH v'Shavyei), however, explains that the grain silo was one of two houses, in one of three courtyards, that comprised the Shiyur in Pumbedisa.)
How, though, do these Rishonim explain how an Amora can argue with Tana'im? The TOSFOS HA'ROSH points out that the text of the Gemara of the Rif and Rambam reads, "Rebbi Yitzchak Omer" (and not "Amar"), which implies that Rebbi Yitzchak was a Tana.
(c) The TUR (OC 392), who also rules like the Rif and Rambam, writes that "one house" suffices as a Shiyur, and he makes no mention of a Chatzer. Indeed, the case of the grain silo in Pumbedisa seems to prove that a single house without a courtyard suffices for a Shiyur.
However, as the BI'UR HALACHAH (OC 392:1) points out, the RITVA (DH Rebbi Yitzchak) states explicitly that Rebbi Yitzchak's words imply that one house without a courtyard is not enough. In the case of the grain silo, the grain silo was like the courtyard, and inside it was a residence, which constituted a house in a courtyard.
HALACHAH: Most Poskim rule like the Rif, that a courtyard with one house suffices (OC 392:1). However, today -- even though it is common to make a Tzuras ha'Pesach together with an Eruv Chatzeros or Shituf Mavo'os for an entire city -- it is not the common practice to leave a Shiyur. Why do we not leave a Shiyur?
1. The MAGEN AVRAHAM (OC 392:2) suggests that we rely on the opinion of Rashi (59a, DH Ir) who maintains that a city is not called an Ir Shel Rabim unless it has 600,000 people in it. Most of our cities do not meet this criterion, and thus it is not necessary to leave a Shiyur.
2. The SHA'AR HA'TZIYUN suggests that even if the Halachah is not in accordance with Rashi's definition of an Ir Shel Rabim, and the presence of a 16-Amah-wide street even without 600,000 people makes a city an Ir Shel Rabim, we still do not need to designate a Shiyur under normal circumstances. This is because even non-Jewish residents can constitute a Shiyur, and there are usually non-Jewish residents living in a city beyond the bounds of the city's Tzuras ha'Pesach. Those residents are considered the Shiyur.
2) WHEN A TECHUM ENDS IN THE "IBUR" OF A CITY
QUESTION: The Gemara differentiates between a situation in which the 2000 Amos that one may walk on Shabbos end in the middle of a neighboring city, and a situation in which the 2000 Amos end at the opposite edge of the city. In the former situation, one may not take a step past 2000 Amos; the city is not judged to be four Amos in its entirety. In the latter situation, the entire city is considered as four Amos and he may walk throughout the city, and the city deducts only four Amos from the 2000 Amos of his Techum on that side.
What is considered the edge of a city? Does it suffice for the person's 2000 Amos to end at the original border of the city, or must his 2000 Amos end after the city's Ibur (the 70-Amah addition to a city, as described on 57a)?
ANSWER: The Rishonim write in the name of the RA'AVAD that it suffices for the person's 2000 Amos to end at the original border of the city. It does not need to end after the Ibur. An Ibur is added to a city only as a lenient measure. It is not counted as part of a city when it would cause a stringency to apply (such as not counting the city as four Amos).