Nedarim, Chapter Five, Mishnah Four
The point of this mishnah is to teach the status of people who may not benefit from one another vis a vis public property. There are two types of public property mentioned in our mishnah and explained in the following mishnah. The first is property which belongs to the people of a given city and the second is property which belongs to all of Israel. Included in the latter category are wells dug by pilgrims from Babylonia coming up to Jerusalem. Hence, the last category is called things which belong to those who came up from Babylonia.
1) [If a man says to his neighbor] Behold, I am herem to you the opposite party is forbidden [to derive benefit from the one who swore].
a) Behold, you are herem to me the one who swore is forbidden.
b) Behold, I am [herem] to you, and you are [herem] to me, both are forbidden.
2) Both are permitted [to enjoy the use of] those things which belong to those who came up from Babylonia [to Jerusalem], but are forbidden [the use of] things that belong to that town.
Section one: The first section of the mishanh basically teaches an additional way in which people may prohibit others from benefiting from themselves, or prohibit themselves from benefiting from others. Each may declare himself herem forbidden property, to the other, or the other herem to himself. In such a way Reuven may prohibit Shimon from benefiting from him, and he may prohibit himself from benefiting from Shimon, or he may even do both in the same statement.
Section two: Although they may not benefit from each other, they may each still use things which belong to those who came up from Babylonia [to Jerusalem]. Each individual Jew is not considered an owner of these things, or even a partner in their ownership. Rather they belong collectively to the children of Israel.
However, town property is owned in partnership by all of the members of the town. Therefore, it is treated like the common courtyard mentioned in mishnayoth one and two. If Reuven and Shimon cannot benefit from one another, then they may not use the commonly owned town property.