Ohalos, Chapter 14, Mishnah 7. This chapter has been dealing with a ziz, a ledge or overhang that juts out from a wall, and with the question of when such a ledge counts as an ohel, a tent, for the laws of tumas meis. The chapter has worked through the case of two ledges, one above the other, each a tefach by a tefach wide, with a tefach of open space between them, and then the case of two such ledges with less than a tefach of height between them. Now the Mishnah asks the next question: what if the ledges themselves are simply too small, so that in one of their dimensions, the width or the length, there is no full tefach at all?
The Mishnah opens: "Ein bohen poseiach tefach," they do not have an opening of a tefach. Neither of the two ledges measures a tefach by a tefach. Once you take the measuring tape and discover that the required width is missing, one conclusion follows immediately: there is no ohel here, and therefore there is nothing to hold back the tumah.
The Ruling: Straight Up and Straight Down
And so the Mishnah continues: "bein sheyeish beineihen poseiach tefach, bein she'ein beineihen poseiach tefach," whether or not there is a tefach of space between the upper ledge and the lower one, the height between them no longer matters. And likewise it makes no difference where the tumas meis is located: "tachteihen," beneath the lower ledge, "beineihen," in the space between the two, "o al gabeihen," or resting on top of the upper one. The halachah is the same in all three cases.
This situation carries a gain and a loss at once. The gain: nothing draws the tumah horizontally outward under the ledge, since no ohel exists to collect it and carry it across. The loss: for that very reason nothing stands in its way vertically either. Hence the rule of "bokaas ve'olah" and "bokaas veyoredes": the impurity pierces upward and pierces downward, cutting right through the ledges in both directions.
To sum up: the moment a full tefach of width and length is not there, nothing remains that could either carry the tumah outward or shut it in. From whatever spot the tumah occupies, it rises and descends along one vertical column, and anything positioned in that column, whether above it or beneath it, contracts tumah.
The Same Law for Two Curtains
A further clause follows: "vechein shtei yeri'os," and so too with two curtains, "shehein gevohos min ha'aretz," which are held above the ground by "poseiach tefach." The term yeri'os will be recognized from Parshas Terumah and from Vayakhel-Pekudei, where it denotes the woven coverings of the Mishkan. Here the image is of two such curtains stretched out flat and level, not draped downward, positioned one over the other. Should each of them measure a full tefach in both directions, with an open tefach separating them, then each one has the standing of an ohel, and the ruling is identical to what the chapter established earlier regarding two ledges that both qualify as ohalos.
That earlier case worked out this way: two ledges, each a tefach wide, with a tefach of height between them. Tumah underneath renders the space underneath tamei, and any vessel there becomes tamei, but the tumah does not rise higher, because the ledge above is an ohel that blocks it. Tumah in the middle space renders any vessel in that middle space tamei. Tumah resting on top of the upper ledge sends tumah straight upward from there. Precisely the same applies to two spread curtains of these dimensions.
And if the curtains are a tefach wide but there is less than a tefach of space between the lower one and the upper one, they follow the earlier halachah of two ledges in that configuration: tumah underneath renders the area underneath tamei; tumah in between, or on top, sends tumah straight up, and even the tumah lying in the middle space passes through the upper one, since the upper one, lacking a tefach of airspace beneath it, does not serve to stop it.
The present Mishnah closes off the series, and the full sequence deserves a look. Where the required poseiach tefach is absent in both the higher and the lower one, it becomes pointless to ask whether a tefach of space separates them. Lose the tefach of width, and the ohel is gone with it. Consequently the location of the tumah, underneath, in the gap, or above, changes nothing: it pierces vertically in both directions. Curtains are subject to precisely what ledges are subject to.
Why the Curtains Are a Chiddush
Applying all this to curtains is quite a chiddush. One would have assumed that a curtain is simply a keli, an item that is mekabel tumah, and an object that itself receives tumah cannot form a partition that arrests tumah. The Mishnah Achronah resolves this by pointing out that once such curtains are stretched out for use as a tent, their standing changes, and they do operate as a chatzitzah that halts the spread of tumah. There are also authorities who understand the case as one where the curtains have been fastened to the ground, and that is what removes them from susceptibility to tumah. In either understanding, the lesson stands: ledges and curtains laid out this way, flat and horizontal, one over the other, share a single body of halachos.