Commentary for Bava Batra 1:2
וְכֵן בְּגִנָּה, מְקוֹם שֶׁנָּהֲגוּ לִגְדֹּר מְחַיְּבִין אוֹתוֹ. אֲבָל בְּבִקְעָה, מְקוֹם שֶׁנָּהֲגוּ שֶׁלֹּא לִגְדֹּר אֵין מְחַיְּבִין אוֹתוֹ, אֶלָּא אִם רוֹצֶה כּוֹנֵס לְתוֹךְ שֶׁלּוֹ וּבוֹנֶה, וְעוֹשֶׂה חֲזִית מִבַּחוּץ. לְפִיכָךְ אִם נָפַל הַכֹּתֶל, הַמָּקוֹם וְהָאֲבָנִים שֶׁלּוֹ. אִם עָשׂוּ מִדַּעַת שְׁנֵיהֶן, בּוֹנִין אֶת הַכֹּתֶל בָּאֶמְצַע, וְעוֹשִׂין חָזִית מִכָּאן וּמִכָּאן. לְפִיכָךְ אִם נָפַל הַכֹּתֶל, הַמָּקוֹם וְהָאֲבָנִים שֶׁל שְׁנֵיהֶם:
And so with a garden, a place where the custom is to fence, he is obligated to do so. [This is the intent: And so with a garden. It is regarded as a place that it is the custom to fence, and one who took a place there is required to fence it.] But in a valley, a place where the custom is not to fence, he is not obligated to do so. [A valley is regarded as a place where the custom is not to fence, and he is not obligated to do so.] But if he (his neighbor) wishes (to fence) he moves into his (own domain) and builds and makes a border-mark on the outside [a sign that the wall is his. The sign is described in the Gemara. He coats a cubit of the top of the wall with lime on his neighbor's side, and not his own, lest his neighbor coat his side, too, and claim that the wall belongs to both. But when he coats it only on his neighbor's side and not on his own, this is a sign that the wall is his; and if his neighbor peels it, it is noticeable (as having been peeled)]. Therefore, if the wall fell, the place and the stones are his. If they both agreed to build, they build the wall in the middle and make the border-mark on the outside, [as a sign that they both built it.] Therefore, if it fell, the place and the stones belong to both.
Bartenura on Mishnah Bava Batra
English Explanation of Mishnah Bava Batra
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.]