Ohalot, Chapter Fifteen, Mishnah Two
1) [With regard to] wooden tablets which touch each other at their corners, and are one handbreadth high off the ground:
a) If there is uncleanness beneath one of them, [a person] touching the second [tablet] becomes defiled with seven-day defilement.
b) Vessels under the first [tablet] become unclean; but those under the second remain clean.
2) A table does not bring uncleanness unless it contains a square of at least one handbreadth.
Section one: This section describes two wooden tablets that touch each other only at one corner. The part that is touching is less than one handbreadth. The two tablets are one handbreadth off the ground, the required amount to be an ohel.
Although they are touching only at their corners, in some ways they are considered to be one ohel. Therefore, if there is uncleanness beneath one of them, a person who touches the other one is impure as if he had been in contact with a dead body.
This is where it gets a bit tricky. If there are vessels underneath the tablets, those under the one where the corpse uncleanness is found are defiled, but those under the other one are not. This seems to contradict the previous line. Albeck explains that since the impurity is not under the second tablet and the second tablet is not touching the first tablet by an amount the size of a handbreadth, the impurity doesn’t spread beneath it. However, we do look at the tablet as if it was overhanging the impurity and one who touches it is impure.
Section two: Tables were often expanded by adding squares, one next to the other, in order to allow more guests (like the leafs we add to our tables). If there is no single square that is at least one handbreadth square, meaning one square to serve as the base of the table, then none of the squares join to form an ohel. This is not considered to be a sustainable ohel (for this concept see 8:5).