Last night my home’s side door’s knob wouldn’t turn its lock for some reason so I had to use the lock above it. You know the kind that sends a bar into a slot in the door frame or even house wall thats often placed above the knob and thats the common standard of front doors, side doors, and backyard doors and any other doors that lead into entering the home at least in Western countries.

So this made me wonder if the whole reason why doors have that secondary lock and not just the door knob’s lock is to function as a backup failsafe and the door knob having a lock also does the same vice versa so that at least there’s one lock if the other malfunctions?

In addition I’m curious does double locking the door actually increase security measures and actually makes the door harder to break in by brute force in addition to making it harder for more sly methods of unlocking? Because I remember seeing a trick where to opena locked door knob, you slide in a credit cards at the slite between the knobt and the door to press that button thing in between around so that you can actually press it open to pull the door out with the knob using your hand or manipulate parts for more advance knobs to unlock it as you press the credit card. In addition I once saw a footage of a burglar on the show COPS trying to unlock the house doors. He’s able to unlock the knob witha bit of difficulty but then has to unlock the top lock and while trying to do so, the owner comes home and the burglar flees like a scaredy cat. The cops then proceed to follow trails to catch him after the owner calls 911 and the whole episode is basically about that burglar. One of the cop even says when investigating the footage told the owner she’s lucky she came home when she did because had she been away fro another hoour or more the burglar would have eventually beena ble to unlock the top and then enter the house to steal whatever he wants.

Also I seen a medieval movie during a raid into a small castle where one door was easily knocked over by ramming a statueand that door was more similar to modern door knob locks even if it didn’t have a knob. While the next door was the kind that inserted bars into t a slot of the door frame or wall (in this case the slot was a few inches deep into the castle’s wall and same witht he bar that locked the door). The protagonist had a much more difficulty ramming that door down with the small statue enough that they decided to drop it and get a larger one and even that heavier statue took more than 5 rams to break the door open, leading to the evil baron to escape.

So with that specific movie example, I also ask if the top secondary lock of a door also helps make it harder to bash the door oepn especially with ramming it with a hard object battering ram style?