Invisible <voi### [at] devnull> wrote:
> The shape of the key aligns a series of pins. By design, exactly one 
> shape of key does this. It's difficult to see how you'd make two 
> unrelated shapes both have this property.

  The simplest lock type is the pin lock: the shape of the key raises each
pin a different amount. Each pin is cut in two parts. If the pin is raised
the correct amount, it will free the pin (as the cut will coincide with the
edge of the lock cylinder). If all the pins are raised the correct amount,
the entire lock will be freed and can be turned.

  In pin locks designed for master keys some of the pins are cut in three
parts instead of two. This means that two differently-shaped keys can open
the lock. When all the locks designed for a given master key have the same
secondary cuts, all of them can be opened with the master key. An individual
such lock can also be opened with the individual key assigned to that lock
(but can't be opened with any other key, except the master key, of course).

  Thus a series of locks with a master keys have all the same pin cut
assignment, plus some of the pins have a second cut, for the lock-specific
key.

  (Of course pin locks are quickly falling out of use, at least in Europe,
because they are so easy to lockpick. Other, more secure lock types are
usually used, but the master key idea is basically the same.)

-- 
                                                          - Warp

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