Ordinarily, people live within a community, and find themselves living and interacting in close proximity with others. Houses face out into a communal courtyard, which themselves share common areas. Similarly, fields share common borders. While it may sometimes be convenient for individuals to share common use of community property, often they will prefer to divide up the property by building a separation between separately owned areas.
The first perek of Massekhet Bava Batra deals with the difficulties inherent in dividing property that is owned by partners and building a separation between them. Is there a standard height and width for such a separation wall? Must it be built out of specific materials? Where exactly should it be built, and what are the rights of the different parties regarding the use of the wall?
The first Mishna teaches that if the two parties agree to divide their courtyard by building a wall, it should be built ba-emtza – in the middle – with each party contributing an equal amount of space, whose area will depend of the type of material that is being used. The Rashash suggests that we do not always make the two parties share equally – if one owns a larger part of the courtyard, he will have to contribute a correspondingly large amount. The Ramah argues – and his position is accepted as the halakha – that the term ba-emtza in the Mishna does not mean that it should be placed in the middle of the courtyard, rather it should be understood to mean that the wall is positioned between the areas that each of them owns, and that they will share equally in contributing to the area that the wall occupies as well as in paying for the expenses of building the wall.