Keilim, Chapter 18, Mishnah 3. Our Mishnah continues the discussion of large wooden furniture and then opens the subject that will occupy the rest of the chapter: the various components of a bed and which of them are susceptible to tumah.
It opens with three large wooden pieces: "hashidah vehateivah vehamigdal", the shidah (a large chest, sometimes built as a wagon), the teivah (a box) and the migdal (a cupboard). Of these the Mishnah speaks when "shenitela achat meragleihen", one of their feet has come off, and it teaches "af al pi shemekablin, tehorin": although they still contain whatever is placed in them, they do not receive tumah.
Recall the basic rule for wooden vessels: in order to be able to receive tumah, a wooden article must function as a receptacle, something that holds what is placed inside it. In our case that condition would seem to be met. Nothing was punctured when the leg came away; the walls and the floor of the piece are intact, and objects placed inside it will still stay inside. Nevertheless the Mishnah rules that these three pieces of furniture are tahor and cannot become tamei. The reason is that they no longer hold their contents in the manner in which such furniture is meant to hold them. Missing a leg, the piece tips to one side and needs to be propped up in order to be used at all. Since it functions only in an abnormal way, we regard it as a broken vessel, and a broken vessel is not susceptible to tumah.
The Mishnah then presents a second case: "veshe'einan mekablin kedarkan" - furniture that does not hold in its usual manner. Here nothing at all is broken. The cart, chest or cabinet is whole, but when it stands upright in its normal position it does not have the capacity to hold forty se'ah, and only if it is tilted somewhat can that volume be contained in it. Concerning such a piece, "Rabbi Yose metamei", Rabbi Yose rules that it is susceptible to tumah.
To appreciate the ruling, we must recall the principle taught earlier in the Masechet: a wooden vessel large enough to hold forty se'ah is not susceptible to tumah, because such an object is too heavy and bulky to be carried about, and an object that is not moved from place to place is not treated as a vessel for these purposes. Rabbi Yose's point is that our piece of furniture does not truly qualify as a vessel that holds forty se'ah, since it reaches that volume only when tipped, which is not its normal manner of use. Not being classified among the immovable, oversized vessels, it stands in the ordinary category of wooden vessels, and therefore it can indeed become tamei.
The parts of a bed
From here the chapter turns to the bed and its components, and it is worth picturing the bed of the time of the Mishnah before reading the words. The bed had a frame made up of a headboard, a footboard and two side boards, and it stood on four legs. Netting was stretched along the length of the frame, with ropes running crosswise, and upon this the bedding was laid. That constituted the bed proper. The legs were commonly set into hollow blocks of wood, and there were wooden supports as well. Some beds also had bedposts and were adorned with metal coverings for decoration.
The Mishnah rules: "menaklitei hamitah vachamor vehipui, tehorin" - the bedposts, the supports and the ornamental metal coverings are tahor. None of these serves as a receptacle, nothing is held inside them, and therefore they are not susceptible to tumah.
"Ein tamei ela mitah umalben" - susceptibility is limited to two items alone. One is the bed itself, that is, its wooden frame. The other is the malben, the hollowed piece of wood into which a foot of the bed is set. Because the malben sits fixed upon that foot, halachah views it as belonging to the bed, and it therefore contracts tumah along with it.
One qualification closes the Mishnah: "umalbenei benei Levi tehorin". A Levi's malben does not receive tumah. When the Levi'im went up to Yerushalayim for their duties in the Beit Hamikdash, their beds stayed behind; what they carried on the road were the malbenin, which they would slip off beforehand. Since in their hands these pieces were never permanently fastened onto the bed's feet, they never counted as part of the bed, and so they remain tahor.