Standard Door Prep Backset & Thickness:
The backset measurement is typically based on the width of the lock stile. Pull tape measure from door edge to the center of the standard prep 2 1/8" diameter bore hole. 2" backset is saved for specific cylinders that can work at that length, if the option is not available for a specific handle or lever set, the set will not work with a 2" backset. Matching backsets are suggested when pairing with additional door controls that are installed separate from each other.
Mortise Lock Backset & Thickness:
A mortise lock set is installed into the pocket hole that has been augered or "hogged" out. The backset measurement is based on the lock case. Pull tape measure from door edge to the center point of the key cylinder and knob spindle hole. Most common backset* is 2 1/2" or 2 3/4". Matching backsets are suggested when pairing with additional door controls that are installed separate from each other.
* Note that there are additional backset offerings for mortise sets, saved for more specialized scenarios. The above two backsets are the most common.