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1. Rebbi Yitzchak discusses the size of the remnant that should be left out of the Eruv.
2. The Mishnah discusses the case of a person who asked his son to make an Eruv for him because he was in a field when Shabbos arrived.
3. There is no reason to place an Eruv in a house that is within 70 Amos of one's city of residence.
4. A person who places an Eruv outside his city extends his Techum in that direction, but he shortens his Techum in the other direction.
5. In the case above (#4), he may lose much more if he lives in a very large city.
A BIT MORE
1. Even though the Tana'im in the Mishnah (59a) give other opinions, Rebbi Yitzchak says that it is enough to leave out one house in one Chatzer. The Gemara is unsure about whether his opinion is based on a tradition or on logic.
2. If he was less than 2,000 Amos away from his home but more than 2,000 Amos away from his Eruv, it is as though he did not make an Eruv. If he was less than 2,000 Amos away from his Eruv but more than 2,000 away from home, his "place of residence" for Shabbos is considered the place where the Eruv is located.
3. This is because that house is considered part of the city, and he has the city's Techum without the need for an Eruv.
4. For example, a person who places his Eruv 1,000 Amos to the east of the city gains 2,000 Amos more to the east, while losing 1,000 Amos to the west.
5. If a person places his Eruv outside his city, he loses the entitlement to treat his entire city as four Amos if none of the city is included within 2,000 Amos from where he places his Eruv. Instead, his Techum is exactly 2,000 Amos from his Eruv into the city and not more.
Next Daf Index to Revach for Maseches Eruvin