Nedarim 45 - When partners fight
If two people jointly owned a courtyard - which was a very common situation - and then they had a falling out, so that each prohibited to himself, by a vow, to benefit from his fellow's property, they have a problem entering their own courtyard. Since the courtyard is not divided, then each portion of it is jointly owned by both of them. Thus, when one treads on the ground, he is partially transgressing the property of his fellow, which is prohibited to him by a vow.
However, Rabbi Elazar ben Yakov says that each one, when he enters the courtyard, enters his own property. When people buy property in common, they divide the ownership of it for use, by area and in time. Exactly which portion and at what time belongs to each is determined by the future event of him entering it. So, when he comes into the courtyard, we say that this is exactly what they had agreed to, and now their agreement is clarified, by his action. This idea of retroactive designation, that is, "I see what happens, and I say that in the past we had actually agreed to exactly this division," is called "breira," or "clarification."
Thus, the argument between the Sages and Rabbi Elazar ben Yakov is in whether or not we can apply "clarification" in this case. The Sages do not permit it, but Rabbi Yakov uses it, because there is no other choice. Should the courtyard be a little larger, so that they can divide, even he would not allow them to use it until they divide.
Art: A Dutch Courtyard by Pieter De Hooch