The distance from the door edge to the center of the existing deadbolt hole. TWO standard values: - 2-3/8”: most common residential. - 2-3/4”: less common, some commercial/heavy residential.
Most new deadbolts are adjustable to fit either. Buy “adjustable backset” to be safe.
Standard residential: 1-3/8” or 1-3/4”. Most new locks fit both, but confirm on the box.