Ohalos, Chapter 14, Mishnah 6. Our chapter deals with the ziz, a ledge or overhang protruding from a wall, and asks under what conditions such a projection has the status of an ohel, so that it spreads tumah in the space beneath it and prevents tumah from continuing upward past it. Our Mishnah takes up a case of two such projections, situated one over the other and lined up with each other. It should be noted at the outset that there are two ways of reading the text of this Mishnah. We will study it first as it is printed in our Mishnayos, and then return to it in the version the Bartenura had before him.
The girsa as it appears in our Mishnayos
Two zizin extend out from a wall, the second positioned over the first. The Mishnah states "yesh bahen posei'ach tefach": each of the two has the surface of an opening of a tefach, meaning a tefach by a tefach. But "ve'ein beineihen posei'ach tefach": the vertical distance from the top of the lower projection to the bottom of the one above it falls short of a tefach. In this reading, though, the space separating the lower ziz from the ground below does amount to a tefach: from the floor to the lower overhang there is a full tefach of clearance.
Now the Mishnah rules. "Tumah tachteyen": if tumas meis is lying underneath the lower ziz, "tachteyen tamei", everything in that space beneath it is tamei. Why? Because that space is a genuine ohel: there is a full tefach of height from the ground to the lower overhang, so the area underneath it forms an ohel of its own, and tumah found there spreads throughout the whole space beneath the ledge. And since an ohel not only spreads tumah but also holds it in, whatever is above the ledge remains tahor. The ohel serves as a chatzitzah against the tumah rising higher.
The Mishnah goes on: "beineihen o al gabeihen", where the tumah is situated in the space separating the two zizin, or resting upon the higher one. In this situation the upper projection creates no ohel over the space below it, for the clearance separating the two overhangs falls short of a tefach, and without a full tefach of height there is no ohel. Two consequences follow from the upper ziz having no status of an ohel: the tumah will not travel horizontally in the space beneath it, and neither will that ziz form a partition to stop the tumah from continuing upward. The Mishnah therefore rules "kenegdo ad haraki'a tamei": the tumah is confined to the column of space directly over it, rising all the way to the heavens.
This is true whether the tumah is resting on top of the upper ziz or lying in the narrow space between the two of them. If it is on top, it is obvious that everything straight above it is tamei. And even when the tumah is in between the two overhangs, the tumah passes right up through the upper ziz, since that upper ziz is not an ohel for lack of a tefach of clearance beneath it.
One may ask: if the tumah is above, does it also travel downward? The implication of the Mishnah is that it does not, and the meforshim explain the reason for this.
The girsa of the Bartenura
We now return to the Mishnah and study it afresh, this time in the version preserved by the Bartenura, a reading shared by numerous other Rishonim. Once again we have two zizin, the one situated directly over the other and aligned with it. "Yesh bahen posei'ach tefach": each measures a tefach by a tefach. "Ve'ein beineihen posei'ach tefach": the space separating the lower from the upper does not amount to a tefach. And now the detail on which everything here depends: the space between the ground and the lower ziz likewise falls short of a tefach. Measure upward from the floor to the lower projection and you will come up short. From the floor to the upper projection, however, a full tefach is there.
In such a case the Mishnah rules: "tumah tachteyen o beineihen, tachteyen ubeineihen tamei". Whether the tumah lies beneath the lower ziz or in the space between the two of them, everything underneath and everything in between becomes tamei.
The reasoning is as follows. Since there is no tefach of clearance from the floor to the lower overhang, that lower ziz is simply dismissed; halachically it is as though it is not there at all. We are left with a single ohel, the upper ziz, since from the ground up to it there is indeed a tefach. Consequently, tumah lying underneath the lower ziz or in the gap between the two spreads through the entire space governed by the upper ohel.
Let us review the Bartenura's version once more. "Yesh bahen posei'ach tefach", each projection is a tefach by a tefach; "ve'ein beineihen posei'ach tefach", the space separating them falls short of a tefach, and so does the space beneath the lower one down to the ground. Should the tumah be "tachteyen", under the lower ziz, or "beineihen", in the space separating the two, the ruling is "tachteyen ubeineihen tamei", both the space below and the space between are tamei. Because the critical tefach of clearance beneath the lower projection is lacking, that projection is halachically disregarded and cannot hold back tumah below it from rising. A single ohel remains, the one formed by the upper ziz, and it conveys tumah to any keili positioned under it, whether that keili sits in the space between the two projections or lower down beneath the first. There is no distinction between the two locations; the whole area is one space.
As one can well imagine, there are many bi'urim and many shitos in this Mishnah, and b'ezras Hashem Yisborach you will surely merit one day to learn them.