Ohalot, Chapter Fourteen, Mishnah Seven
1) If they did not have a width of a handbreadth, whether there is a space of a handbreadth between them or whether there is not, if there is uncleanness beneath, between or above them, the uncleanness cleaves upwards and downwards.
2) A similar [rule applies] to two curtains, [the lower one of which is] one handbreadth high off the ground.
Section one: If neither projection is a handbreadth wide, then no ohel is formed. Therefore, no matter where the impurity is found, it only travels upwards and downwards, but it does not spread to the sides.
Section two: The final clause of this chapter notes that the same scenario described in mishnayot 5-7 can occur if two curtains are placed one above the other, and the bottom one is at least one handbreadth above the other.