Keilim, Chapter 19, Mishnah 6. This mishnah continues the chapter's discussion of the mizran, the length of cloth or fabric that was fastened onto a bed, and then turns to the bed frame itself and asks a question about spare parts: when a bed's boards are swapped for new ones, which set of boards determines the status of the bed?
The Mizran Wrapped Around the Bed
The mishnah opens: "Mitah shekarach lah mizran venaga bahen hamet, temei'in tum'at shiv'ah." A bed had a mizran wound around it, and then either the bed or the mizran touched a corpse. Both of them contract the seven-day tum'ah.
The reason is that once the mizran is wrapped onto the bed, the two of them function as a single unit. Contact with the met by either part draws in the other, and both are tamei with tum'at shiv'ah.
The mishnah goes on: "Pirshu, temei'in tum'at shiv'ah." Should the mizran later be unwound and lifted off the frame, neither piece sheds what it acquired: the bed keeps its seven-day tum'ah and so does the mizran. Nothing here resembles a utensil that shattered and was thereby purified. A mizran is a keli in its own standing, and the bed is a keli as well; when the two come apart, all that has happened is that two independent utensils have gone their separate ways, each holding on to the tum'ah it received.
The identical pattern governs the lighter grade of tum'ah. "Naga bahen hasheretz", a sheretz came into contact with one of the two, and the ruling is "temei'in tum'at erev": both become a rishon letum'ah, tamei only until nightfall. And should they afterward be pulled apart, "pirshu, temei'in tum'at erev", neither of them is released; each carries that one-day tum'ah by itself.
The Bed Frame and Its Notches
To follow the second half of the mishnah, one needs a picture of how a bed was built. The frame consisted of the two long sideboards, the shorter boards at the head and foot, and the legs. The legs were bored with openings, notches into which the boards were seated. If a person replaced the boards with new ones, the notches would ordinarily have to be reshaped to accommodate them, and once that was done the original boards could no longer be fitted back in.
The mishnah teaches: "Mitah shenitlu shetei arukot shelah." A tamei bed had its two long sideboards removed. Note that these boards were not broken; they were simply taken out, and the owner could reinsert them whenever he wished. As long as the bed sits disassembled it is not tamei, since it is not a functioning keli. But if he puts those same intact boards back into their places, the bed is whole again and it returns to its tum'ah.
Now the mishnah develops the case. A fresh pair of long boards was fashioned and fitted into the legs, and the openings already bored there were left exactly as they had been: "Ve'asa lah chadashot", he made new ones for it, "velo shinah et hanekavim", and he did not recut the notches. The consequence is that the legs can still receive the first pair of boards, which are intact and on hand.
Suppose now that the newly fitted boards snap: "nishtabru chadashot", and the ruling is "temei'ah". The bed can still return to its tum'ah, since the owner has only to retrieve the first pair of boards and slide them into the notches that were never reshaped. The moment he does so, the very same bed stands before us again, complete and usable, and the tum'ah it once bore comes back along with it.
If instead it is the earlier pair that snaps, the mishnah rules "vihyeshanot, tehorah": the bed is tahor and never recovers the tum'ah it had, "shehakol holech achar hayeshanot", since the whole question is decided by the first set of boards.
The reasoning runs as follows. When we ask whether a bed that has come apart can be brought back to its tum'ah, we look at which parts made up the bed at the moment the tum'ah reached it. Should that first pair of long boards still be around, nothing worse than a dismantling has taken place, the pieces can be put back together, and the tum'ah returns. Should that pair have snapped, the bed now has the status of a utensil that broke, and a broken utensil has no route back to the tum'ah it once bore.