Ohalos, Chapter 14, Mishnah 2. This chapter is occupied with the ziz, a ledge or projection jutting out from the wall of a house, and the question of when such a ledge counts as an ohel, a tent, capable of carrying tumah from what lies beneath it into the house. Our Mishnah takes up two cases: a ledge above a doorway, and a ledge above a window.
A ledge above a closed door
The Mishnah begins: "ziz shel gabei pesach" (a ledge above a doorway). According to the Bartenura, the case under discussion here is a doorway whose door is shut. Because of that, this ledge "meivi es hatumah" (brings the tumah) only "bifosayach tefach," that is, only when it measures a tefach by a tefach.
The contrast with the preceding Mishnah is sharp. There we learned that a ledge above a doorway need not have a full tefach by a tefach, and the reason was that the doorway in question stood open. Here, where the door is closed, the ledge does need its tefach.
But this raises an obvious difficulty. If the door is shut, what difference does the size of the ledge make? Do we really mean that once the ledge has a tefach by a tefach, the tumah finds its way inside the house? The answer is no. Even a ledge of a full tefach cannot push tumah past a closed door. What it does accomplish is this: it spreads tumah to the area beneath it, to whatever is standing in the doorway itself, on the outer side of the shut door. Other explanations of these words are offered by the meforshim, but this is the way the Bartenura understands the Mishnah.
A ledge above a window
The Mishnah now shifts subjects entirely. Until this point we were dealing with ledges above doorways; from here we deal with ledges above windows.
It is worth recalling the shiurim for windows that were taught in the chapter before this one. The size a window must have in order for tumah to travel through it depends entirely on why the window was built. Where a window was fashioned to admit daylight, the shiur is "k'oivi hamakdeiach," the width of the drill that was in use in the Beis Hamikdash. And where such a light-window was in the process of being blocked up, its owner having started to seal it and then abandoned the work, the shiur becomes "rum etzba'yim": two fingerbreadths in height, with a thumb's breadth across.
With these two windows in mind, this is how the Bartenura renders the words of the Mishnah. Whether we are speaking of a ledge "shel gabei chalon" above a window that was begun to be sealed and left unfinished, whose shiur is a height of two fingers by a thumb's width, or of a ledge above a window made for light, whose shiur is "melo makdeiach," the thickness of that drill, in either case the ledge itself is "kolshehu," of any size at all. Even if it lacks a tefach, tumah lying beneath it is carried into the house. In other words: tumah under the ledge, an open window above it, and the house is tamei. Here too there are other approaches among the meforshim, but this is the Bartenura's reading.
Reb Yosi's view
Reb Yosi rules: "meloyo." As he sees it, a ledge over a window becomes an ohel only once it measures as much as the window beneath it is required to measure. Take a ledge above a window built for daylight: it has to extend outward from the wall as far as a melo makdeiach, which is exactly the shiur at which such a window counts as open. Reb Yosi's principle is that whichever measurement renders that window large enough to admit tumah into the house is the same measurement demanded of the ledge's projection. And in the case of a window that someone started sealing, whose shiur is two fingerbreadths by a thumb, the height there being counted as two fingers, that becomes the required outward reach of the ledge as well.
A second approach, based on the Tosefta
The meforshim bring a different way of learning the Mishnah, grounded in a Tosefta. When the Mishnah speaks of a height of two fingers, it is not describing how far the ledge extends out of the wall at all. Nor is it identifying which type of window we are dealing with. Rather, it is measuring how high above the window the ledge is positioned.
Visualize the arrangement. A ledge stands over a window, and suppose the window is an ordinary one, built for regular household use rather than for daylight, so that its shiur is a tefach by a tefach. Here the ledge transmits the tumah inward even if the ledge is under a tefach, on condition that it is not set higher than two fingerbreadths above the window. Now suppose instead a daylight window, whose shiur is a melo makdeiach (in this clause of the Mishnah the measurement belongs to the window, not the ledge). For such a window the ledge is subject to no shiur at all, not in its distance above the window and not in its breadth. However far above the window it may be, and however slightly it may stick out of the wall, it carries the tumah into the house.
Why should we be so strict in this case? The Mishnah Acharonah offers a lovely insight. Where the window is made for practical use, a ledge above it is simply part of the arrangement; people set things down on such a ledge, and it draws no special attention to itself. But over a window made for light, what is a ledge doing there? Its presence announces itself as a roof over the window, an ohel in its own right, and therefore we treat it more stringently.
And Reb Yosi says: "meloyo." On this second approach, Reb Yosi is disputing the ruling concerning the window made for light. He refuses to accept that the ledge has no shiur at all, that its height above the window is irrelevant and its width is irrelevant. If the ledge stands over a window made for light, the very shiur of that window applies to the ledge as well: it must not be positioned more than a melo makdeiach above the window, and it must project from the wall at least a melo makdeiach.
To restate this second reading clearly, since the challenge is to fit it precisely into the Mishnah's words: a ledge over an ordinary window, one made for putting things through and general use, brings tumah so long as it is within two fingerbreadths above the window. A ledge over a window made for light has no shiur at all, because here it is self-evident that the ledge is there as an ohel, and it makes no difference how high above the window it stands or how little it juts out from the wall. That is the position of the Chachamim. Reb Yosi answers no: even with a window made for light there are limits placed on the ledge. To qualify as an ohel it must fall within the same measurements as the window itself, no higher than a melo makdeiach above the window, and projecting from the wall at least a melo makdeiach.