Keilim, Chapter 28, Mishnah 8. This chapter deals with cloths and garments and the ways in which they receive tumah. Our Mishnah takes up a case of two items joined together: a patch that had already become tamei, which is then sewn onto a garment in order to close a hole in the middle of it.
The principle at work is "kol hamechubar latamei, tamei": whatever is attached to something tamei is drawn into its tumah. If the patch is genuinely part of the garment, the garment becomes tamei just as the patch is. But this depends entirely on the quality of the attachment. The patch has to be joined to the garment in a real and functional way, and not merely tacked on. The Mishnah therefore spells out how much stitching creates a true chibur, a halachic connection.
"Telayah al habeged meruach achat, eino chibur." If the tamei patch was stitched onto the garment along one side only, while the other three sides hang loose, this is not considered a chibur. The patch flaps about and does not truly serve the garment, and so the garment remains tahor.
"Mishtei ruchot, zo keneged zo, chibur." If, however, it was sewn down on two sides that face one another, one side and then the opposite side, that is a chibur. Being anchored at two opposing edges holds the patch in place over the hole, and the garment now becomes tamei together with the patch.
"Asao kemin ga'am, Rabbi Akiva metamei, vachachamim meta'harim." What if the stitching was done in the shape of a ga'am, the Greek letter gamma, whose form is like the letter L: one line of stitching along the bottom and one running up the adjacent side? Here the two sewn sides are not opposite one another but perpendicular. Rabbi Akiva holds that this counts as an attachment, so the garment becomes tamei. The Chachamim hold that it is not a chibur, and the garment stays tahor.
The Mishnah stated that a patch stitched on one side alone is no chibur and leaves the garment tahor. Rabbi Yehuda now limits that ruling to a particular kind of garment. "Amar Rabbi Yehuda: bameh devarim amurim? Betalit." That applies to a talit, a cloak of the sort we still use: essentially a flat sheet of fabric that a person lays over his shoulders, throws one end up over the other, and wraps himself in. Such a garment has no fixed top and bottom. It is draped and worn in many different ways, so a patch stitched on a single side will sometimes sit above the hole and sometimes below it, shifting back and forth. Since it cannot be relied upon to cover the hole, it is not a chibur.
"Aval bechaluk, milema'alan chibur, milematan eino chibur." A chaluk, a shirt, is different, because it cannot be worn upside down. The arms go through the sleeves and the head through the neck opening, so the garment always has a definite top and bottom. Therefore, if the single line of stitching is above the hole, the patch lies flat and is held in place, and it will always fall over the hole and cover it. That is a chibur, and if the patch was tamei the shirt becomes tamei as well. But if the stitching is below the hole, the patch will curl and flop over, leaving the hole exposed, and such a patch is not considered attached to the garment.