"A Skeleton key is a type of master key in which the serrated edge has been filed down so that it can open numerous locks. The term derives from the fact that the key has been reduced to its essential parts.
A TARDIS key was the key to the exterior door of a TARDIS. They could be found in a variety of shapes and designs. Whatever the key's shape and size, and whatever form the TARDIS' outer plasmic shell, the key would fit a lock somewhere in the structure."
