cellio | daf bit: Bava Batra 2

We now begin the last of the "Bavas", a trio of tractates related to torts and property claims. (I've been told that they were originally one very large tractate.)

Bava Batra begins by talking about fences. If joint owners of a courtyard (that is, the owners of the houses bordering it) agree to build a wall, they should make it down the middle and each should contribute half of the building materials. If in the future the wall falls, it is assumed that the place it occupied and the stones/bricks belong to both equally. In a place where fences are not customary, like in a cornfield, one who wants to build a fence cannot compel the other to share in the effort. If they don't agree, the one who wants the fence must build it set back from the property line, making a facing on the other side. If the wall later falls, we assume that it will all still be on his property and it will be clear that the land and the materials belong to him. (2a)

Today's daf is 4, and it's in the middle of the g'mara discussing this mishna.