Kelim, Chapter Eighteen, Mishnah Three
1) If one of the legs was missing from a chest, a box or a cupboard, even though it is still capable of holding [things], it is clean, since it cannot hold [things] in the usual manner.
a) But Rabbi Yose says: it is susceptible to impurity.
2) The poles of a bed, its base, and [its] covering are clean.
3) Only the bed itself and its rectangle [frame] are susceptible to uncleanness.
4) The rectangle frames of the Levites are clean.
Section one: According to the first opinion, if a leg is missing the box, chest or cupboard is no longer susceptible to impurity, even if it can still hold things in it. The reason it is clean is that it can no longer be used in its usual fashion.
Rabbi Yose says that as long as the box, chest or cupboard is usable, it is still susceptible.
Section two: The mishnah now begins to discuss the various parts of beds. The poles, the base and the bed covering are all clean, because they are no truly necessary for the bed’s functioning.
The poles stand one at each end of the bed and they hold up the canopy.
Section three: The rectangular frame is put into the empty area of the bed and the bedding is placed on it. Since the frame and the bed are actually slept upon, they are susceptible to impurity.
Section four: The Levites would take their rectangular frames with them when they went to Jerusalem and sleep on them without putting them into a real bed. Since they were not used with a bed, they are not susceptible to impurity.