Kelim, Chapter Nineteen, Mishnah One
1) One who dismantles a bed in order that he might immerse it and [while doing so] touches the ropes remains clean.
2) When does the rope begin to constitute a connective with the bed?
a) As soon as three rows of meshes of it have been knotted.
3) And [if another rope was tied to this one] and a person touches it:
a) If from the knot inwards he becomes unclean;
b) But if from the knot outwards he remains clean.
4) As to the loose ends of the knot, any one that touches that part which is needed for it becomes unclean.
a) And how much is needed for it? Rabbi Judah says: three fingerbreadths.
Section one: If while dismantling a large bed in order to immerse it in a mikveh to purify it a person touched the ropes of the bed, he is not thereby defiled. This is because the ropes of a dismantled bed are not considered impure. As we shall see, the ropes are considered to be impure only when attached to the bed.
Section two: Beds in the time of the Mishnah were made by tying ropes tightly around the bed frame. The mishnah now asks when are ropes considered connected to the bed such that their purity/impurity status matches that of the bed. The answer is that as soon as three rows have been tied onto the bed. It would seem that at that point the bed offers at least minimal support.
Section three: If one ties another rope to the rope that is already tied to the bed, the new rope is considered connected from the knot and inwards. That portion of the rope is impure, and if the bed is impure it will defile him if he touches it. But from the rope and outwards, it is pure and it will not defile him.
Section four: The first three fingerbreadths of the loose ends of the knot were needed to tie the knot. Therefore, they are impure. Anything beyond that is not impure.