Durham Cathedral
Sanctuary Knocker

Those who 'had committed a great offence' such as murder
in self-defence or breaking out of prison, could rap the
knocker and would be given 37 days of sanctuary within
which they could try to reconcile with their enemies or
plan their escape.  The current knocker is a replica -
the original is displayed in the Cathedral
Treasures Exhibit inside the building.

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Durham Cathedral
Durham Cathedral (Wikipedia)