A door chain, security chain, security door chain, or St. Louis Double is a small chain on a door for security purposes. It is a type of lock that is often used along with other types of locks to secure a door.
A door chain makes it possible for someone on the inside to open the door while still making it difficult for someone outside to force his or her way inside.
A door chain also makes it so that someone who picks the lock from the outside still cannot get inside, as the door chain cannot be easily circumvented while the door is open.
To bypass a door chain, the door chain must be snapped. For this reason, door chains are usually made out of some durable metal. Examples include brass and steel.
UPVC/PVCU front doors are generally unsuitable for retro-fit security devices. Not only is the material not strong enough to support devices fitted with steel screws unless secured into the internal metal framework, but such changes to the original design may invalidate an existing warranty or possibly damage the integral locking assembly.[1]
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