Originally, it is said, only doctors had door knockers; other houses and flats had bells. They were introduced later as decorative elements in brass or, on basement doors, iron.

The temptation to add new knockers should be resisted, particularly on doors to flats from the internal common stair, where they are inappropriate and noisy for adjoining occupants.

Care should be taken not to damage the zinc conduit during any re-plastering works, and it is important to avoid exposed wiring and cranks becoming paint-bound during the redecoration of common stairs.