Interior door locks can provide anything from a sense of privacy for a room's occupants to major security for valuable stored items. When choosing a lock for your interior doors, be sure to consider the main purpose. It's not necessary to spend the money for an ultra-secure lock if you're merely looking for a way to stop people from walking into an occupied bathroom.
Privacy locks offer the lowest level of security for an interior door. This type of lock operates from the inside of a room with the occupant pushing a button or twisting a knob to activate it. No keys are involved. Such locks often have an emergency release in the form of a small hole in the exterior knob. A small metal bar can be stuck into the hole to open the lock in the event someone accidentally presses the button, leaving the room locked with no one inside to open it.
Keyed entry door locks work for interior doors where a medium level of security is required. This type comes with a lock built into the door knob or handle. Most handle sets also come with a deadbolt for a higher security level. Keyed entry locks are sufficient for interior doors like those for offices and classrooms.
For interior doors to rooms needing a high level of security, such as apartments with interior entryways, a deadbolt lock is an option. Deadbolts usually serve as a second lock accompanying a keyed door handle. For most situations, a single-cylinder deadbolt requiring only a key to open it from outside is sufficient. For more security, a double-cylinder deadbolt requires you to use a key to lock and open both sides of the door. For safety, it's best to avoid the double cylinder in any residential situation, as it can hinder escape in a fire.
For extra-sensitive interior areas that require a high level of security clearance, you can install a keyless door lock. Some locks of this type require a security code to enter, while others must be preprogrammed with fingerprints of those allowed access. The user places his finger on the lock and gains entry if allowed.