Lock backset
The Guild of Architectural Ironmongers publication 'Glossary of architectural hardware terms' defines a backset, a lock backset (or sometimes called a door backset or the backset of a door) as 'The horizontal distance measured from the outside face of the forend to the centre of the keyway (or cylinder), or to the centre of the follower or to both.'
Here the forend refers to the visible part of the lock on the door surface once fitted, the keyway is the hole or aperture in the cylinder where the key goes ( also just called a key hole) and the follower is the hole in the door into which the lock cylinder or spindle is fed.
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