Keys fit to be tied? No just a name for the newest type of security system on the lock scene! Around 800-900 AD the first all-metal locks were invented. These locks differed from their ancient predecessors not only in the materials used but also in their mechanism. Instead of using pins they utilized a new system known as wards, or obstructions on the interior of the lock. The key has to have the right cuts for it to pass the obstructions which then allows it to turn and throw the bolt or latch (much like jumping the hurdles in a race). It has been rumored that the ancient Greeks and Romans have been using the warded lock well before this time so I suppose some acknowledgement must be given to them as well.
One problem with the early warded lock was the simplicity of their mechanism. As a result locksmiths over time created ever more complex wards making it difficult for one key to open another lock (see figure above). Some examples have been keys require to have the key cut into a cross (popular with many churches at the time to be sure!) as well as having the post of the lock to be made into a clover design (see figure below).
Many of the locks from the 14th. through the 18th. centuries were for the most past based on artistic accomplishment. Very ornate and beautiful and ranging in sizes they have largely remained the same in mechanical terms. It was not until the around the early nineteenth century that bits were invented for the next step of evolution.