Kelim, Chapter Twenty-Seven, Mishnah Seven
1) If a piece of cloth was woven to the size of three by three handbreadths, and then it contracted midras uncleanness, and then he completed the rest of the piece, and then one removed a single thread from the original part, it is free from midras uncleanness but is still unclean from contact with midras uncleanness.
2) If he removed a thread from the original part and then he finished the whole cloth, it is still unclean from contact with midras uncleanness.
Section one: When he makes a piece of cloth that is three by three, it is susceptible to midras uncleanness. It then is defiled so that it has midras. When he makes the rest of the cloth, the whole piece of cloth has midras uncleanness. But then when he removes one string from the original three by three, the whole cloth loses midras uncleanness because only the original piece actually had midras, and it is no longer there. However, the cloth still has uncleanness by virtue of its contact with something that had midras. For a similar type of mishnah see 19:5.
Section two: In this case, first he made the three by three and it became impure and then he reduced it by a single thread, and then completed the rest of the cloth. Even though the cloth was reduced in size before he completed it, thereby losing its midras impurity, the whole cloth is impure by virtue of contact with midras. This is because the three by three piece of cloth is considered as having contact with itself before it was reduced. After it was reduced, although it can no longer have midras impurity, it still has impurity by contact with midras, and when he completes the cloth, the whole thing has impurity by contact with midras.