The deadbolt lock has long been “the lynch-pin of physical security,” and remains so today. Deadbolts are strong mechanisms that resist both forcible entry and lock-picks, so they are usually near the top of any physical security expert’s list of the right tools for the job. To the average consumer a deadbolt is a deadbolt; the lock snicks into place and it’s locked, right?
It may be locked, but it might not be fully secure. Most homeowners use a single cylinder deadbolt on their exterior doors, which is often an acceptable choice. A single cylinder deadbolt is a lock that has a keyhole on one side and a thumb flip or toggle switch on the other. When the toggle is flipped toward the door jamb, the door is locked, and can be opened either by disengaging the toggle (flipping it back toward the door), or using the key on the other (exterior) side.
By comparison, a double cylinder deadbolt has exactly the same mechanism, but the “throw” (the actual bolt itself) can only engaged or disengaged by using a key, and both sides of the lock utilize keys. A double cylinder lock provides an extra layer of security because no one can unlock it from the inside or outside without the key. This is of particular importance for securing doors with windows in them; a thief cannot smash in the glass and reach through to manually unlock the door. It will not unlock without a key in one or the other cylinder of the lock.
In most customer-heavy businesses a glass door is preferred for security, so you can see who is coming and going easily. However, those same doors become hazards if secured with a single cylinder lock. When installing either variety of deadbolt, it is important to make sure that the throw (bolt) is at least 1” long so that it sinks deeply enough into the jamb to secure the door.