The best home security involves a layered approach. No single aspect of your comprehensive security apparatus should be considered sufficient on its own. Ideally, smart security depends on multiple factors. All must be carefully maintained and occasionally reevaluated. Some measures involve modern technology. A video security surveillance system is obviously important for the most sophisticated protection. Modern video surveillance allows homeowners to monitor conditions on the home front from afar, using Web-based applications. An electronic alarm system is also helpful. But at least one important aspect of home security is far from modern. Its technology was developed literally thousands of years ago: Your door lock. Research indicates that up to one-third or more of burglaries involve entry through an open or unlocked door. Obviously, as a front-line defense against intrusion, door locks are useless if you fail to engage them. But even scrupulous door- and window-locking may be insufficient. Research also shows that a significant proportion of unlawful entries involve access with a key. And that brings up the question: When is it prudent to change your door locks? At least four specific instances warrant changing out your locks. Moving In Buying a new property or moving into a new rental can be an exciting proposition. But don’t forget security in the excitement of moving. You have no way of knowing who may possess keys to your property’s existing locks. The only way to be secure is to change out locks and carefully control who has access to keys. Changing Relationships Whenever you experience a change in a relationship, and someone you formerly trusted with a key is no longer necessarily welcome in your life or your home, it’s probably wise to change your locks. Even if you remain on good terms, this eliminates any worries that you may experience unexpected—and unwelcome—visits in the future. This applies not only to significant others, but to old roommates or former friends who may have obtained a key copy. Your Locks Are Old Like virtually anything else, locks are delicate mechanisms that can wear out. If operating your lock becomes tricky, consider replacing your old lock. Chances are an upgraded model will offer better security, at any rate. You Lost Your Key(s) Sooner or later, most of us will lose our keys, if only temporarily. While you may eventually recover them, don’t take any chances. Instead of hoping for the best, take this opportunity to install new locks and take better care with your new keys.