Keilim, Chapter 10, Mishnah 7. Until now the chapter has been occupied with tzamid patil, the tightly sealed earthenware vessel that acts as a shield so that tumah cannot reach whatever lies inside it. Our mishnah introduces a completely different mechanism of protection: not a seal, but a tent within a tent. When objects are placed inside a small ohel that itself stands within a larger ohel containing corpse tumah, the inner tent blocks the tumah and the objects inside it stay tahor.
For the inner tent to do this work, two conditions must be met. First, its walls and its roof may not themselves be keilim, since a keli receives tumah rather than blocking it. Second, the inner tent may not be propped up by a keli. If the roof of the small tent is held in place by a vessel that supports it, the roof loses its power to keep tumah out. The whole mishnah is an exercise in identifying, in a pair of ovens, exactly what is resting on what.
The mishnah opens: "Tannur yashan betoch hechadash", an old oven standing inside a new one. An earthenware oven becomes a keli only once it has been finished, meaning it has been fired and heated to the requisite degree. So the terms here are precise. The outer oven is new: it was never fired, it is unfinished, and therefore it is not yet a keli at all. The inner oven is old: it has already been heated and completed, and it is a full keli. In other words, the inner oven is a vessel and the outer oven is not.
"Usrida al pi hayashan": a flat earthenware baking plate, an item that never acquired the status of a keli, has been laid over the opening of the old oven. Here the mishnah supplies its test: "Nittal hayashan, usrida nofelet", picture the old oven being taken away. Would the plate come down? If it would, then the old oven had been bearing its weight all along, and the ruling is "hakol tameh", whatever lies within the unfired oven contracts tumah. Oven and plate together offer no defense against the corpse tumah filling the larger tent, since this smaller tent leans upon a vessel, and once a tent leans on a keli it forfeits its power to bar tumah.
Note that the plate is not plastered onto the old oven; it merely lies over it. That is why it also cannot help as a tzamid patil: covering is not sealing. In principle the vessel here could have shielded its contents in the manner of a sealed container, but in this case the seal was never made. So neither route to protection is available, and the contents are tameh.
"Ve'im lav, hakol tahor." Suppose instead that taking the old oven out of the picture would leave the plate exactly where it sits. That shows its support never came from the old oven at all, and then the contents of the inner new oven remain pure. Plate and unfired oven together constitute a tent unto itself, resting on nothing that carries the status of a vessel, and such a tent guards what it covers from the tumah of the tent around it. The whole arrangement succeeds only because the outer oven has never been fired: an oven that was never completed is not yet a keli, and that is precisely what qualifies it to serve as a barrier.
The mishnah now flips the picture: "Chadash betoch hayashan", this time the unfired oven, which has not attained the status of a vessel, sits inside the fired one, an oven that does count as a keli, with the earthenware plate (again nothing that qualifies as a vessel) laid across the opening of the outer oven, "usrida al pi hayashan". On the surface it appears that a vessel is bearing the plate's weight. The mishnah nonetheless rules: "im ein bein chadash lisrida" a "pote'ach tefach", if the space separating the top of the inner unfired oven from the plate above it does not amount to an open handbreadth, then "kol shebechadash tahor", the objects within the inner oven stay pure.
What is the reasoning? The old oven is not the plate's genuine base. Remove it, and the plate would simply come down onto the rim of the inner oven directly below. Its true resting place, then, is not a vessel at all: it would settle upon the unfired oven and become one with it. And once the plate belongs in this way to an oven that has never become a keli, the two of them function as an ohel in its own right, a tent that holds the surrounding tumah outside. The contents keep their taharah, and that is so even where no plaster fastens the plate to the oven's rim. Sealing is demanded only when one relies on tzamid patil, protection through a tightly closed cover. A tent, whose defense lies in preventing tumah from entering, does its work even when parts of it are left open.
This is also why the measurement matters. Should there be a full tefach of open space between the plate above and the new oven below, tumah spreads into that space, exactly as it would pass through a handbreadth-sized opening in any tent. The ruling of purity therefore holds only where less than a handbreadth separates the plate from the new oven beneath it.