During the time of the Mishna, common burial practice was for families to arrange for burial caves. Every family would purchase a rocky area that they would dig out, creating an entrance area surrounded by a series of caves, one for each household in the family. In each cave, burial niches – called kukhin – were chiseled out of the rock. Each of the kukhin would open into the cave, and the dead body would be placed in it after which the kukh would be sealed with rocks, plaster, etc.
The Mishna on our daf discusses the sale of an ordinary burial cave, which is meant to serve as a family burial plot or catacomb. The Tanna Kamma of the Mishna teaches that an ordinary cave must offer enough room to build a chamber of four amot wide by six amot long, with room for eight kukhin, three on either side and two opposite the entrance. Each of these kukhin must be four amot long, six tefaḥim wide and seven tefaḥim high.
The rishonim explain that the size of these areas is important for both practical and halakhic reasons. The Rashbam explains that a length of four amot is needed to fit a normal sized person in a casket – ordinarily a casket made of stone. The Ritva and Ramah point out that this includes room to seal the kukh with dirt, rocks, etc. The need for a height of seven tefaḥim is related to the rules of ritual purity and defilement. When a body is in a tightly enclosed space, the tumah that emanates from the dead body “leaps” beyond the enclosure, putting kohanim at risk to become ritually defiled should they walk above the body. If, however, there is a pote’ah tefaḥ – an opening the size of a handbreadth above the body in a closed area, the tumah will be held in that immediate area, and ritual defilement will not be spread. It is, therefore, important to have enough space to contain the tumah.