Kelim, Chapter Eleven, Mishnah Two
1) Every metal vessel that has a name of its own [is susceptible to impurity,]
2) Except for a door, a bolt, a lock, a socket under a hinge, a hinge, a clapper, and the [threshold] groove under a door post, since these are intended to be attached to the ground.
Section one: As long as a metal vessel has a name of its own, and its name is not based on another vessel, it is susceptible to impurity.
Section two: The exceptions are various parts of doors. These are not susceptible to impurity because they are meant to be attached to the ground, and any vessel that is attached to the ground is not susceptible to impurity (see 8:9).