Kelim, Chapter Twenty, Mishnah Six
1) If a sheet that was susceptible to midras uncleanness made into a curtain, it becomes clean from midras uncleanness but it is still susceptible to corpse uncleanness.
2) When does it become insusceptible to [midras] uncleanness?
a) Bet Shammai says: when the loops have been tied to it. .
b) Bet Hillel says: when it has been attached.
c) Rabbi Akiva says: when it has been fixed.
Section one: A sheet used for bedding is susceptible to midras uncleanness because it is laid upon. But if he makes it into a curtain, it is no longer used for lying upon, and therefore it is not susceptible to madras impurity. It will however be susceptible to corpse impurity as are all vessels made of cloth.
Section two: There are three opinions as to the exact point at which the sheet ceases being a sheet and becomes a curtain. According to Bet Shammai, once loops have been attached to the sheet, it is a curtain. To put it abstractly, it is a curtain if it looks like a curtain even before it functions as such. Bet Hillel says that it must be attached to the opening for it to be a curtain. A curtain must function like a curtain. Rabbi Akiva postpones the time a bit more. It must be fixed to the opening with nails. Abstractly speakingit must be set up as is a normal curtainwith loops, attached to the opening with nails. Before that, it is still just a sheet.