Mishnà su Ezechiele 26:9

The sheath of a sword, a knife or a dagger, the case for scissors, or shears or a razor, the case of make-up and the make-up box, the stylus case, the compass case, the tablet case, the case for a board, a quiver and a javelin case all these are susceptible to uncleanness. The case for a double flute is susceptible to uncleanness if the instrument is put in from above, but if it is put in from the side it is clean. A case for flutes Rabbi Judah says it is clean because these are put in from the side. The covering of a club, a bow or a spear is clean. This is the general rule: that which serves as a case is susceptible to uncleanness, but that which is merely a covering is clean.