פירוש על בבא בתרא 1:1

השותפין שרצו לעשות מחיצה – we are speaking of a courtyard where there is no law of division in it, where there lacks by the way four cubits for each one of the partners. For this reason, it (i.e., the Mishnah) taught " "שרצו/that they wanted, which implies that especially, two of them want [to make a partition] but neither one of them is able to force his fellow, for if in a courtyard which has a law of division, each one of them is able to force his fellow to divide, but now, it comes to teach us that since they wanted to divide it, and each one of them had taken possession of the direction that is entitled for him according to what they had compromised, they build the wall in the middle, and this one gives from his portion one-half of the place the thickness of the wall and also this one.

Introduction
The first mishnah of Bava Batra deal with neighbors who share a courtyard. The mishnah is concerned with the building of a wall to separate the neighbors and with the ability of one neighbor to force the other neighbor to share the costs of building the wall.

גויל – stones which are not trimmed or fixed.

Mishnah One
1) If two partners wish to make a partition in a courtyard they build the wall in the middle.
2) In a place where the custom is to build of unshaped stones, or of hewn stones, or of half-bricks, or of whole bricks, so they should build it—everything is according to local custom.
a) [If the wall is made of] unshaped stones this one supplies [from his property] three handbreadths, and this one supplies [from his property] three handbreadths.
b) [If the wall is made of] hewn stones this one supplies [from his property] one and a half handbreadths, and this one supplies [from his property] one and a half handbreadths.
c) [If the wall is made of] half-bricks this one supplies [from his property] two handbreadths, and this one supplies [from his property] two handbreadths.
d) [If the wall is made of] whole bricks this one supplies [from his property] one and a half handbreadths, and this one supplies [from his property] one and a half handbreadths.

גזית – stones which are trimmed and planed with a saw.

Explanation
In the time of the mishnah most homes did not have openings to the street but rather would open onto a common courtyard. The courtyard was used for all sorts of purposes and was the common property of the owners of the houses surrounding it. Our mishnah states that if the two partners wish to build a wall separating the courtyard they should build the wall in the middle and when they do, they should build the wall with the materials customary used in their place. There are four kinds of building materials mentioned in the mishnah, and the mishnah lists each one.
Clauses 2a through 2d all state how much of a person’s property he should allocate for the thickness of the wall. This space will depend on the thickness of the building materials. Unshaped stones are (on average) 6 handbreadths wide and therefore each partner must allocate three handbreadths of his property for the building of the wall. Hewn stones are only 3 handbreadths wide and therefore each partner allocates one and a half handbreadths. A whole brick is 3 handbreadths wide, and therefore each partner allocates one and a half handbreadths. Half bricks are one and half handbreadths wide. In order to make a wall with them they would use two half-bricks, placing mortar in between the two. The total width of the wall would be four handbreadths, and therefore each partner would allocate two handbreadths.

כפיסין – a small brick/girder which is a one-half brick which is a handbreadth-and-a-half (i.e., one handbreadth equals 3.65 inches or 9.34 cm) and a brick is three handbreadths.

Questions for Further Thought:

• Mishnah one: If one partner wished to build with half-bricks and one partner with whole bricks, and the custom of the land was to use hewn stone, what type of wall should they build?

• What laws in modern society are similar to these types of laws? How are they different?
If two partners wish to make a partition in a courtyard they build the wall in the middle.
In a place where the custom is to build of unshaped stones, or of hewn stones, or of half-bricks, or of whole bricks, so they should build it everything is according to local custom. [If the wall is made of] unshaped stones this one supplies [from his property] three handbreadths, and this one supplies [from his property] three handbreadths. [If the wall is made of] hewn stones this one supplies [from his property] one and a half handbreadths, and this one supplies [from his property] one and a half handbreadths. [If the wall is made of] half-bricks this one supplies [from his property] two handbreadths, and this one supplies [from his property] two handbreadths. [If the wall is made of] whole bricks this one supplies [from his property] one and a half handbreadths, and this one supplies [from his property] one and a half handbreadths.
The first mishnah of Bava Batra deal with neighbors who share a courtyard. The mishnah is concerned with the building of a wall to separate the neighbors and with the ability of one neighbor to force the other neighbor to share the costs of building the wall.
In the time of the mishnah most homes did not have openings to the street but rather would open onto a common courtyard. The courtyard was used for all sorts of purposes and was the common property of the owners of the houses surrounding it. Our mishnah states that if the two partners wish to build a wall separating the courtyard they should build the wall in the middle and when they do, they should build the wall with the materials customary used in their place. There are four kinds of building materials mentioned in the mishnah, and the mishnah lists each one.
Clauses 2a through 2d all state how much of a person’s property he should allocate for the thickness of the wall. This space will depend on the thickness of the building materials. Unshaped stones are (on average) 6 handbreadths wide and therefore each partner must allocate three handbreadths of his property for the building of the wall. Hewn stones are only 3 handbreadths wide and therefore each partner allocates one and a half handbreadths. A whole brick is 3 handbreadths wide, and therefore each partner allocates one and a half handbreadths. Half bricks are one and half handbreadths wide. In order to make a wall with them they would use two half-bricks, placing mortar in between the two. The total width of the wall would be four handbreadths, and therefore each partner would allocate two handbreadths.

הכל כמנהג המדינה – "הכל"/everything includes a place where they have the practice to make a partition with the sprouts of vine and the branches of trees that they make according to the custom, as long as the partition is wide enough so that neither one of them cis able to see that of his fellow for visual damage is called damage, and the height of the wall and the partition is not less than four cubits.

זה נותן שלשה טפחים – that the wall of untrimmed stones requires an additional handbreadth more than the wall of hewn stones because of the tops of the stones that protrude outward which are not smooth.

ובכפסים זה נותן טפחי – that the thickness of the wall of the one-half/small/girder brick is a handbreadth greater than the width of a wall of full brings because he places a small brick/bond-timber on one side and a small brick/bond-timber on the other side, which are three handbreadths and a handbreadth in the middle in which they place there plaster to attach them, but the wall made from bricks, one places a full brick along the thickness of the wall which is three handbreadths and there is no need for plaster in the middle.

המקום והאבנים של שניהם – this tells us that even though the stones fell into the domain of one them, or alternatively, that one of them first removed them into his domain, you might have thought that it belongs to the other – for the claimant must produce the evidence. But this comes to tell us that this is not the case.