Niddah, Chapter 8, Mishnah 2. This chapter deals with the kesem, a bloodstain that a woman discovers after the fact, and asks in which circumstances the stain renders her tmei'ah. The guiding principle is that a stain counts only where it can reasonably be traced to the beis haturpa, the place from which uterine blood emerges. Having applied that principle to stains on the flesh, the Mishnah now applies it to stains found on her clothing.
"Ra'asa al chalukah": she found a stain on her garment. Here the dividing line is her belt. "Min hachagor ulematah," if the stain is on the portion of the garment from the belt downward, "tmei'ah," she is impure, since that part of the garment lies where blood from the uterus could have reached it. "Min hachagor ulema'alah," if it is from the belt upward, "tehorah," she is pure, because that area is out of reach.
Next the Mishnah turns to the sleeve. "Ra'asa al beis yad," she noticed a stain on the arm portion, "shel chalukah," of that same garment. Here the test is how far the sleeve can be drawn. "Im magi'a," if it reaches, "kneged beis haturpa," to a position facing the place from which uterine blood emerges, "tmei'ah," she is impure. "Ve'im lav," but if the stained spot on the arm portion lies too high or too far to have arrived at that position, "tehorah," she remains pure.
The final case is one in which no section of the cloth can be excluded. "Haysa poshat'to umiskasah bo balailah": she takes the garment off and spreads it over herself as a blanket through the night. In that situation, "kol makom," any spot at all, "shenimtza bo kesem," on which a stain is discovered, "tmei'ah." The Mishnah supplies the explanation itself: "mipnei shehu chozer," the cloth gets turned around and bunched up in every direction while she sleeps under it, so any part of it may well have come to rest facing the beis haturpa.
"Vechein bafalyon," and this identical ruling governs the covering worn on the head through the night. That article too gets twisted and shifted in all directions, and therefore a stain located anywhere upon it leaves her tmei'ah.
The Mishnah thus measures a stain on clothing by exactly the same standard as a stain on the body: not by where the cloth happens to sit at the moment of discovery, but by whether that piece of cloth could have found its way opposite the beis haturpa. Where it could, she is tmei'ah; where it could not, her taharah is undisturbed.