The backset is the distance from the edge of the door to the center of the 2-1/8" bore hole. In the U.S. there are two common backsets for residential and commerical locks, 2-3/8" and 2-3/4". Another way to understand the measurement of the backset is to measure from the edge of a door to the center of the locks keyway. The image seen below is a standard hole preparation for deadbolts and knob/lever locks.
You can also identify your backset by measuring you deadbolt or latch.
Below is examples of a mortise lock. The backset is also measured from the edge of the door or the locks faceplate to the center of the locks mortise cylinder hole or keyway. The mortise lock has additional dimension which are critical for proper identification and replacement and should be included in your identification process.
Call your local Society of Professional Locksmith member when seeking advice or service!
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Backset image on this page was created and published on Directdoorhardware.com
Tom Lynch, CRL