A skeleton key (also master key or pass key) is a key that can open all of the locks in a set.
For example, in an office building, each employee would have a key that only opens his/her office, and the manager or owner would have a key that opens all of the offices in that building.
More complicated setups are also possible. For example, if there are cleaners in the office building, and each cleaner only cleans one floor, the cleaner for a each floor could have a sub-master key that opens only the offices on that floor, while each employee still has a key that opens only his/her office and the manager still has a key that opens all of the doors. The exterior doors might also be configured so that they can be opened by any employee key, cleaner key, or the manager's master key. The more complicated the setup, the more important it is to use locks that have a large number of key possibilities. Disc-detainer locks are especially suitable for master keying due to their large number of key possibilities.