Overview
- Lock cylinders use small pins and springs that the correct key lines up to allow turning, and leaving a key inserted keeps those parts under tension which can cause long-term wear.
- Many homes now use double cylinders with a notfall function that lets the lock be turned from outside even if a key is stuck inside, so an inserted key does not reliably block entry.
- Doors with glass panels or sidelights are especially vulnerable because a burglar can break the glass, reach in, turn a key left inside, and open the door.
- Insurance companies typically only require proof that a door was properly locked after a break-in, so leaving a key inside is not needed to support a claim.
- Security advisers recommend replacing or upgrading hardware—for example with an internal thumbturn or a panic lock that opens from inside—to keep convenience without weakening security and to prevent avoidable long-term damage to older locks.