Ohalot, Chapter Twelve, Mishnah Three
1) [In a case where] the board is placed over the mouth of an old oven projects from either [end] to the extent of one handbreadth but not from the sides:
a) If there is uncleanness under one end [of the board], vessels [under] the other end remain clean.
b) Rabbi Yose declares them unclean.
2) A betach does not bring uncleanness.
3) If there was a projection (ziz) in it:
a) Rabbi Eliezer says: it [still] does not bring uncleanness.
b) Rabbi Joshua says: we look at the betach as if it is not there, and the projection above brings uncleanness.
Section one: In this case the board extends over the oven in one direction, for instance, north-south, but not east-west. The first opinion holds that this is not sufficient to convey impurity from one side to the other. The impurity would have to travel through the oven and since the oven cannot become impure from the outside it blocks the impurity from going to the other side. In mishnah one the board hung over the oven from all sides, and therefore the impurity could travel from one side to the other.
Rabbi Yose holds that the door does not prevent the impurity from traveling from one side to the other.
Section two: A betach is some sort of projection that juts out from a window both inside and out. Maimonides interprets it as something used to climb on. The mishnah says that it doesn’t bring uncleanness from the outside into the house because the wall found in between forms a break.
Section three: In this case there is another type of projection sticking out from the house that is found on top of the betach (in Hebrew this is called a ziz). The impurity is found underneath the betach and the question isdoes the ziz convey impurity into the house?.
Rabbi Eliezer holds that the betach separates the impurity from the ziz and therefore the ziz doesn’t convey impurity into the house.
Rabbi Joshua holds that we look at the situation as if the betach wasn’t there. Therefore, the ziz is found to form an ohel over the impurity. The house to which the ziz is attached will also be impure.