A single cylinder deadbolt should be mounted on a solid wood core door when there is no breakable glass within 40 inches of the locking device.
A double cylinder deadbolt lock, which is key operated from both the interior and exterior, should be used when there is glass within 40 inches of the locking device
Caution: the use of a double-cylinder deadbolt lock can be a fire hazard. While occupied a key should be left in the lock and all children should be trained in the use of the lock and all other fire prevention methods.
Basic Lock Criteria
Both of these locking devices should meet the following basic criteria:
The bolt must extend a minimum of 1 inch and be case hardened or contain a hardened insert.
The connecting screws must be at least one-fourth in diameter and screw into solid metal stock - not screw posts.
The connecting screws that hold the lock together must both be on the inside. There must be no screw heads on the exterior of the door.
The cylinder guard must be tapered or spherical in design to make it difficult to grip with pliers. It must be of solid metal and it is preferred that the guard turn freely.