Double Cylinder Function: The deadbolt is thrown and retracted by a key from either side. As you will not be able to unlock this from the inside without a key, please check your local fire code to ensure this is allowed before purchasing. Product Features: Equipped with a full 1" throwbolt Fully adjustable latch to fit all standard door preparations (2-3/8" to 2-3/4" Backsets) ANSI/BHMA Grade 3 Security Patented side locking bar technology offers improved security by protecting against lock bumping, an attack technique used to defeat conventional pin & tumbler locks. Two keys will be shipped per lock. Three or more items keyed alike will ship a total of 6 keys SmartKey®: This technology allows you to re-key your lock in seconds with 3 simple steps and features stainless steel racks, pins, and side locking bar for increased strength and security. Specifications: Backset: Adjustable for 2-3/8" or 2-3/4" Crossbore 2-1/8" Edge Bore 1" Latch Face 1" x 2-1/4" Door thickness: 1-3/8"-1-3/4" doors standard. Cylinder: 5-pin SmartKey cylinder, cannot be keyed alike to a 6 pin cylinder. Bolt: 1" throw Additional Functions: 660-S: Single Cylinder Deadbolt 665-S (This Model): Double Cylinder Deadbolt