The second perek of Massekhet Bava Batra, Perek Lo Yahpor, focuses on the care that a person must take within his own property so that he does not cause damage to his neighbor’s property.
The first Mishna lists a number of things that are perfectly reasonable things to do on one’s own property, but that they must be kept at least three tefahim from the wall that divides the properties. Thus the Mishna teaches that a person cannot dig a hole or a ditch close to the boundary line, nor a canal for the transfer of water or a pool used for washing clothing.
The commentaries discuss the source of these rules, since the Gemara appears to offer two separate potential problems. On the one hand, it appears that the concern with these structures stems from the presence of water, and fear that the moisture may cause problems. On the other hand the Gemara implies that the excavation itself is a source of concern, lest it undermine the foundations of the neighbor’s property.
While Tosafot points to the moisture as the main concern, claiming that the digging only creates the potential for damage, the Rashba argues that the digging itself creates problems of stability and is the main reason for the enactment requiring three tefahim of separation. The Ritva quotes the Re’ah and the Ge’onim who say that both reasons are applicable, and either one would be sufficient reason to create this rule.
Aside from excavations, the Mishna lists other normal field uses that must be kept away from the fence marking the border, ranging from things that create heat like lime and fertilizer to things that may damage the wall like plowing and planting right next to it. Similarly, the Mishna teaches, an oven must be kept away from the wall, three tefahim for the kalya, which is four tefahim from the opening of the oven.
The kalya is the part of the oven that is the widest; according to Rashi, it is the base of the oven which tapers to a smaller opening at the top.