This refers to the measurement from the edge of the door to the center of the main bore of an installed lock. To obtain this measurement, hook a measuring tape on the edge of the door and measure right to center of the keyhole. The most common backset for modern residential-grade locks is 2-3/8". The standard backset for commercial-grade locksets is 2-3/4." If you are buying a replacement lock, this is the most important dimension to match.