Kelim, Chapter Thirteen, Mishnah Six
Our principle teaches and illustrates the general principle that a vessel is considered to be made of the material which makes up its function part. This principle is of importance because the rules governing wooden and metal vessels are different.
1) Wood that serves a metal vessel is susceptible to impurity, but metal that serves a wooden vessel is clean.
2) How so? If a lock is of wood and its clutches are of metal, even if only one of them is so, it is susceptible to impurity, but if the lock is of metal and its clutches are of wood, it is clean.
3) If a ring was of metal and its seal of coral, it is susceptible to impurity, but if the ring was of coral and its seal of metal, it is clean.
4) The tooth in the plate of a lock or in a key is susceptible to impurity by itself.
Section one: If the functional part of a vessel is made of metal, it is susceptible to impurity even if its ancillary parts are made of wood. But if the functional part is made of wood, it is not susceptible even if its ancillary parts are metal.
Section two: The mishnah now illustrates this through several examples. The first example is a lock. The clutches are the functional part. Therefore, even if only one of them is of metal, the lock is susceptible.
Section three: The main part of the ring is the ring itself. Therefore if it is of coral, the ring is clean (coral is never susceptible to impurity), but if it is of metal, it is susceptible.
Section four: Since the mishnah discussed locks, it adds that if the tooth in the plate of a lock is separate from the lock or key, it can still be susceptible to impurity, because it is considered a vessel in and of itself.