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1. A private city that grew to a population of 600,000 people still can have a citywide Eruv.
2. A public city that decreased to less than 600,000 people may not make a citywide Eruv.
3. There is a dispute about the number of residents that must be left out of the citywide Eruv in this case (#2).
4. The Gemara explains that a ladder may be considered like a wall, but not like a doorway.
5. The Gemara explains that a ladder may be considered like a doorway between two courtyards.
A BIT MORE
1. It may have an Eruv provided that it does not have a street that is sixteen Amos wide (according to Rashi).
2. It may not make an Eruv unless it leaves part of the city out of the Eruv, since there is a possibility that it will revert back to the type of city which cannot have an Eruv.
3. Rebbi Yehudah: Fifty residents must be left out of the city Eruv in order to show that they must monitor the growth of the city, lest it begin too large to have an Eruv. Rebbi Shimon: Leaving three courtyards out of the Eruv, each containing two houses, suffices.
4. Accordingly, in a city where there is an opening on one side and a ladder leaning on the wall on the other side, it is not considered as though a major thoroughfare runs through the city, and a regular Eruv may be made.
5. In a case where a ladder is placed between two courtyards, the people in the two courtyards are considered to be in one domain, and thus they may make one Eruv together. If they choose to remain separate, they may make two separate Eruvin.
Next Daf Index to Revach for Maseches Eruvin