Common door locks fall into one of three categories: cylinder locks, rim locks, and mortise locks.
Cylinder locks are the most common type of door lock. They are designed to fit into pre-cut holes in standard doors.
Rim locks are an older type of lock that is seldom seen anymore. They can sometimes be useful to add an additional layer of security to an existing door lock. Instead of going through a hole in the door like a cylinder lock, rim locks are screwed onto the interior surface of the door.
Mortise locks are similar to cylinder locks in that they are mounted through the door, but instead of mounting through a standard hole like a cylinder lock, mortise locks are mounted into much larger pockets in the door. Because mortise locks are larger, they provide additional protection against forced entry.
Spring Bolts vs. Dead Bolts
Common low-security locks use spring bolts with angled edges. The spring applies a constant force to push the bolt outwards. When the door closes, the angled head of the bolt slides against the door jamb. This pushes the bolt against the spring and back into the door frame. As the door closes fully, the bolt head slides into a hole in the door jamb and stays there until the door knob is turned again to retract the bolt so that the door may be opened.
Dead bolts offer higher security. A dead bolt lock has no spring action. It engages or disengages the bolt only based on the action of the user. This is more secure against prying attacks, because the entire lock mechanism — not just a spring — holds the bolt in it’s locked state.
Single Cylinder vs. Double Cylinder Deadbolts
A single cylinder lock has a key only on the outside. The lock can be undone from the inside by turning a lever. A double cylinder lock requires a key to open the door even from the inside.
The weakness of single cylinder locks is that, if the attacker can break a window in or near the door, they can reach a hand in and open the door from the inside. Double cylinder locks protect against that, but create a security risk in that you or a family member may not be able to exit the building in a hurry — which usually means in case of fire.
Recommendations
My recommendation is to use a single cylinder deadbolt mortise lock on a door with no windows — and to have no windows near your door. The only window necessary should be a good peephole mounted in the door.
For additional security, you can add a deadbolt rim lock. The best of these use vertical bolts. Vertical bolts are more resistant to prying.
