פירוש על בבא בתרא 1:2

וכן בגנה – this is what he said, "וכן בגנה"/and likewise in the case of a garden, undefined, as a place where it was they had the custom to fence it in.

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. However, in a valley, where it is not customary to build a fence, they cannot obligate him to do so.
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. Therefore if the wall falls, the place and the stones belong to him.
If they acted with each other’s consent, they should build the wall in the middle and put a finishing on both sides. Therefore if the wall falls, the place and the stones belong to them both.
Mishnah two is a continuation of mishnah one. It continues to discuss the procedures for building a wall to separate property.
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 person’s 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 Albeck’s explanation and not the explanation given in the Talmud.]

אבל בבקעה – undefined, it is like a place where they had the custom to not fence it in, and we don’t require it, other than if his fellow wanted to fence it in, he moves it back to his own part

ועושה לו חזית – a sign for recognition that the wall is his. And the sign is explained in the Gemara (Talmud Bava Batra 4b): he spreads the top of the wall a cubit with plaster to the side of his fellow, but not to his side lest his fellow all spread from his side. And he says that the wall belongs to both. But when he spreads to the side of his fellow, he doesn’t plaster on to his side. It is a sign that the wall is his, but if his fellow peels/scrapes It off, it is known as peeled.

ועושין חזית מכאן ומכאן – to announce that both of them made it.