Bava Batra, Chapter One, Mishnah Two
Mishnah two is a continuation of mishnah one. It continues to discuss the procedures for building a wall to separate property.
1) The same is true with a garden: in a place where the custom is to build a fence, they can obligate him to do so.
a) However, in a valley, where it is not customary to build a fence, they cannot obligate him to do so.
2) But if he wants to [build a fence] he must gather into his own portion and build, and he puts a finishing on the outside of the wall.
a) Therefore if the wall falls, the place and the stones belong to him.
3) If they acted with each others consent, they should build the wall in the middle and put a finishing on both sides.
a) Therefore if the wall falls, the place and the stones belong to them both.
Mishnah two begins by comparing a garden to a courtyard. The mishnah states that a garden is similar to a courtyard in that if it is customary to build there a fence, one partner can force the other to share in the costs and allocated space. However, in section 1a we learn that a valley is different. Since it is not customary to build there a fence, neither can force the other to do so. If one partner wishes in any case to make a fence he must do it in his own area and put a finishing on the wall so that it will it will not look unfinished nor ugly in the eyes of the one who did not want a fence, nor will it be rough and potentially damage the other persons animals. Since he has built the wall on his own property and he alone paid for it, if the wall should fall, both the place and the materials belong to him. If however, the two partners built the wall together and shared in its costs and space, they should finish both sides of the wall. If the wall should fall they will share the space and the materials.
[Please note that I explained the idea of finishing the wall according to Albecks explanation and not the explanation given in the Talmud.]