Keilim, Chapter 7, Mishnah 6. This chapter deals with the kirah, the portable stove, and with the ways in which tumah reaches it. The previous mishnah taught a case in which the feet of the kirah project outward past its side. So long as those protruding feet remain within three fingerbreadths of the kirah, they are considered part of it, and they can contract tumah through the air space that lies between them. The present mishnah asks the obvious follow-up question: exactly which air space are we talking about, and how do we mark out its boundaries?
The Mishnah opens with the question itself: "Keitzad mesha'arin otan", how do we measure that air space of the feet?
Rabban Shimon ben Gamliel supplies the answer. First, "notein et hakaneh beineihen": one lays the kaneh, a measuring rod, across the space separating the two feet. Then comes the ruling that follows from that line. "Min hakaneh velachutz tahor": whatever lies past the rod, on its outer side, stays tahor. "Min hakaneh velifnim": whatever lies on the inner side of the rod is tamei, "umekom hakaneh tamei", and the strip of space that the rod itself occupies counts with the inner side.
Here is how the measurement is carried out in practice. One measures three fingerbreadths along each of the two protruding feet, starting from the kirah, and marks that point. The kaneh, a rod that serves here as a straight ruler, is then laid down between the feet, running from the three fingerbreadth mark on the one foot to the three fingerbreadth mark on the other. The two feet and the rod between them now form a triangle, and that triangle is the air space under discussion.
The region beyond that line, everything lying on the far side of the rod and outside the triangle, is tahor. Should a sheretz come to rest out there, no tumah reaches the feet, since that area is no longer counted as part of the kirah.
The region on the near side of the line, the whole enclosed area of the triangle, is tamei, and the narrow space taken up by the rod itself belongs to that inner region as well. A sheretz that lands anywhere within it renders both the feet and the kirah tamei.
In a single stroke, then, the Mishnah turns the abstract measurement of three fingerbreadths into a clear geometric boundary: draw the line between the two marks, and the question of tumah is settled by whether the sheretz landed inside that triangle or outside it.