Here are the basic locks: Exterior entry locksets have a large, rectangular body that slides into a mortise (a cavity carved into the edge of the door made to receive the lock mechanism). A mortise lockset contains the workings for the knob, lever, or grip handle, latch, and deadbolt in a single unit. With a mortise set, the knob generally is interconnected with a security deadbolt. Mounting a mortise lockset calls for fairly tricky carpentry work.

Cylinder locksets have a rounded body designed to fit into intersecting holes bored into the door. The deadbolt bar, which slides into a corresponding opening in the doorjamb, is the main source of your security.

Doors: Exterior doors tend to be heavier than interior doors for both security and weatherproofing reasons. Interior doors also can use less secure, keyless locks, which can be set by the press or turn of a button. The doorknob package with this type of lock is an interior passage lockset.

Other locks: A rim lock, or surface deadbolt, is the type of deadbolt lock that is independent of the doorknob. It's installed in the inside of the door and the bolt fits into a corresponding sleeve attached to the inside door frame.

The basic padlock, a familiar sight on gates, lock doors and even luggage zippers, is a portable key-operated lock built around a steel loop that clicks into the lock's body. The level of security a lockset offers depends on its construction. Any type with only a key in the knob or handle is only marginally secure; a burglar can easily foil it. For more security, a deadbolt should be installed with at least a one-inch throw-which means it should extend a minimum of one inch beyond the door's edge and be made of case-hardened steel.

A double-cylinder deadbolt requires the use of a key from both sides of the door. This is the safest type to use for doors with windows. A knob, lever or grip handle operates a cylindrical lockset. To provide better security, it can be paired with a deadbolt.

Hours: Many locksmiths offer 24-hour emergency services. Expect to pay more money for off-hours services, though. If your problem can wait, call your locksmith during normal business hours.

Lock-repair services: Cutting new keys is a simple, inexpensive process. Most hardware stores also offer this.

Door won't close shut? With time and temperature shifts, houses settle and move up and down. This can dramatically affect how a latch or deadbolt matches up with the hole on the doorframe. This can be a simple do-it-yourself project. Simply file the hole bigger. If your door knobs or handles were installed after 1960, they likely have standardized hardware. So replacing the damaged hardware is easier and less expensive. For doors installed before 1960, it may be worthwhile to try to repair the hardware (especially with old antique doors).

If your door will not lock, try a simple test before calling a locksmith. Open the door and turn the lock key while the door is open. If it works, then the hole in the door frame may be incorrectly aligned with the deadbolt. A simple fix is to file the hole opening wider. If the door still doesn't lock with the door open, the problem may be more serious, and you may want to call a locksmith.

-- Tips courtesy of HomeAdvisor.com