Ohalot, Chapter Fifteen, Mishnah Three
1) [With regard to] jars standing on their bottoms or lying on their sides in the open air and touching one another to the extent of a handbreadth:
a) If there is uncleanness beneath one of them, the uncleanness cleaves upwards and downwards.
2) When does this rule apply? When the [jars] are clean.
a) But in the case where they were unclean or one handbreadth high off the ground, if there is uncleanness beneath one of them, what is beneath all becomes unclean.
Section one: The jars mentioned referred to here are made of earthenware and therefore cannot be defiled from the outside (only if a source of impurity enters their airspace are they impure). The jars are found outside, not in a house or other type of ohel. They are standing on their bottoms or on their sides such that there is not a handbreadth of space beneath them. These jars do not form an ohel. Therefore, even if they are touching each other by the length of a handbreadth, impurity doesn’t spread from one to the other. Even within one jar it doesn’t spread because it is not in an ohel. It only travels up and down.
Section two: The above is true only if the jars are clean. If they are unclean they do not prevent the impurity from entering within. Once within, it is as if the impurity is found in an ohel and it spreads from one jar to the other. Similarly, if the jars are at least a handbreadth off the ground, they form an ohel and the impurity will spread from one jar to the other because the contact point is at least a handbreadth long.