According to Leviticus 15:4, any object on which a person who has had a discharge sits is impure. The Rabbis taught that things that are not made to sit upon or are not fit to sit upon, do not receive impurity. Our mishnah discusses two types of stools and whether or not they can receive impurity.
A bride’s stool from which the covering-boards have been taken: Beth Shammai pronounces it [liable to become] unclean, And Beth Hillel pronounce it not [liable to become] unclean. Shammai says: “Even the framework of a stool [by itself is liable to become] unclean.” An ordinary stool was made of four legs held together by four boards (the framework), on which were placed covering boards for sitting. A bride’s stool had, in addition, three upright covering boards (also called ‘covering-boards’), against which the occupant leant. The question in our mishnah is with regards to the status of a bride’s stool that does not have its special covering boards. According to Beth Shammai, since this stool can still be sat upon, it can receive impurity. According to Beth Hillel, since it does not fulfill its intended function as a bride’s stool, it cannot receive impurity. Shammai, again taking the strictest position, holds that even a simple framework with no coverings can receive impurity. Since one could potentially sit on the framework, it is considered enough of a stool to potentially become impure.
A stool which has been set in a baker’s trough: Beth Shammai pronounces it [liable to become] unclean, And Beth Hillel pronounces it not [liable to become] unclean. Shammai says: “Even one made therein [is liable to become unclean].” If someone took a stool and put it into a kneading trough, in order to put flour or dough upon it, Beth Shammai would still rule that it is a stool and therefore susceptible to impurity. Beth Hillel rules that the stool is now part of the trough, and since the trough can’t become impure, neither can the stool. The reason that the trough can’t become impure is that it was not made for sitting. Shammai rules that even a stool that was made from the outset as part of a trough, and therefore was never intended for sitting, can become impure. Although it was not intended for sitting, since one could sit on it, it can become impure.