(Mishnah): When partners want to divide their field by making a wall, they should build the wall in the middle (with both of them paying half of the cost).

In a place where the custom is to build with unshaped rocks; with carved rocks; with half bricks (1 1/2 tefachim thick), or with full bricks (3 tefachim thick), they should construct it with them. Everything follows the custom of the place.

For a wall of unshaped rocks, each gives 3 tefachim of their land;

For a wall of carved rocks, each gives 2 1/2 tefachim of their land;

For a wall of half bricks, each gives 2 tefachim of their land;

For a wall of full bricks, each gives 1 1/2 tefachim of their land;

Therefore, if the wall fell down, they both have equal share of the place and the stones.

(Gemara) - Question: (The Mishnah taught that if the partners wish to make a wall, they may do so.) How could the later Mishnah teach - 'They may only divide a courtyard if each will receive 4 amos'.

Answer: There, they do not agree that it should be built (as one of them does not wish to do so); here, they both agree to build it.

Answer #2: One could even say that here, they also do not both agree, but if one wishes to build, we require the other one to agree.