Keilim, Chapter 24, Mishnah 17. Our mishnah takes up kupot, boxes or baskets, that have been fastened to one another so that the two of them now serve as one utensil, and it asks how tum'ah behaves in such a case.
The governing principle is this: when two items have been joined into a single vessel, we identify which of the two is the essential one, the main body of the utensil, and the status of the whole follows it. Should that principal part be tamei, everything attached to it is tamei along with it; should it be tahor, the rest is tahor as well. The mishnah teaches us how to recognize that principal part when the utensil before us was formed by joining one box to another.
The mishnah opens with a count: "shalosh kupot hen," three distinct rulings are laid down here for boxes that have been fastened together.
A worn box patched onto a sound one
"Mehuha shetilayah al haberiyah": a box that had worn out was used as a patch for a box that was still sound. The owner set the one inside the other and fixed the two together. In such a case, "holchin ahar haberiyah," we go after the sound box, since that is clearly the significant one. If the sound box is tamei, the worn out box that was attached to it becomes tamei too; if the sound box is tahor, the worn out box is tahor as well.
A lesser box joined to a bigger one
"Ketanah al hagedolah": in this instance neither box is worn out, both are equally serviceable, and they differ only in their dimensions, the smaller having been attached to the bigger. Here the deciding factor is capacity: "holchin ahar hagedolah," the combined vessel takes its tum'ah or its taharah from the bigger box.
Two boxes that are equal
"Hayu shavot, holchin ahar hapenimit": if the two boxes were of the same size and also in the same condition, so that neither sturdiness nor size gives us a way to rank them, we follow the inner one. Whichever box was placed on the inside is treated as the more significant of the two, and the status of the whole utensil is determined by it.
Ribbi Shimon: the pan of a scale on a kettle
Ribbi Shimon contributes a further illustration: a "kaf moznayim," one of the pans belonging to a balance scale, "shetilayah al shulei hameiham," which somebody employed to mend the underside of a kettle. In the situation he describes, tum'ah rested upon the pan while the kettle itself stood in taharah, and we want to know what becomes of the kettle once the two have been fastened together.
His ruling is "mibifnim tamei, mibachutz tahor." If the pan was fixed to the inside of the kettle's base, the kettle turns tamei like the pan; if the pan was fixed to the outside, the kettle stays tahor. The reasoning follows the rule already given: the pan and the kettle are equal in size and in condition, so we go after whichever of them is on the inside. When the tamei pan is on the inside, both the pan and the kettle are tamei; when the tahor kettle is the inner one, the kettle and the pan alike are tahor.
A pan attached to the kettle's side
"Tilayah al sidah": the pan was set in as a mend in the wall of the kettle rather than beneath its floor. In that event the mishnah rules "bein mibifnim bein mibachutz," whether the mending was laid against the inner face or against the outer face, the verdict is "tahor." A pan serving as a mend in the kettle's wall is the lesser of the pair in size, and therefore the inner/outer test is set aside and we return to the rule of the bigger item, which here is the kettle. The kettle carries no tum'ah, so the combined vessel, kettle with pan, remains tahor.
So our mishnah gives us a clear order of priority for boxes and vessels joined into one: sturdiness first, then size, and only when the two are truly equal do we look to see which of them sits on the inside.