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I noticed that my side door is equipped with two locks. The upper lock is a single-cylinder deadbolt, while the lower one is a lever lock. Both locks are operated using the same key.

So what's the purpose of the up one? It seems redundant?

Outside:

enter image description here

Inside:

enter image description here

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  • 1
    Are there not two locks like this on your front door and other external doors? If not, there should be!
    – FreeMan
    Jun 7, 2023 at 12:48
  • my front door is a smart lock:) @FreeMan
    – Wayne Li
    Jun 8, 2023 at 13:17
  • Having spent an entire career in IT (and having a number of smart products in my own home), I stand by my original statement. ;)
    – FreeMan
    Jun 9, 2023 at 17:18

3 Answers 3

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This is an extremely common setup.

The top lock is a deadbolt. A deadbolt, within the limits of design (some types of locks are easier to pick than others) and installation (a lock is in a weak door or a weak frame then it will only be as secure as the weakest part) is generally very secure. In general, getting past a locked deadbolt requires some violent action - breaking the glass to reach in and turn the latch (which is actually a very real problem with glass doors or with doors that have a glass panel next to the door, but for fire safety you don't want to have a double-cylinder (key needed inside or out)) or cutting into the door or frame.

The bottom lock is, well, not much of a lock. It doesn't provide much security. It is enough to keep pets and (friendly) kids out. But it does very little to stop a determined adult. It is often trivially easy to pop open the lock with a screwdriver or sometimes even just a credit card. A telephone guy I worked with once referred to a flat screwdriver as the "universal key" because it works quite well to get into any telephone closet that doesn't have a deadbolt. In fact, I actually recommend disabling the lock mechanism on this type of lock, particularly if it is designed such that you can accidentally lock yourself out. With a typical (there may be exceptions) deadbolt you can't lock yourself out. But with a lock like this you can lock yourself out while at the same time doing little to prevent the bad guys from getting in.

In other words - use the top lock for security, and use the bottom lock just for the handle to open/close the door.

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    installed correctly a deadlatch like the one on seen here will not open to a shimming attack.
    – Jasen
    Jun 6, 2023 at 0:37
  • @Jasen True. But very often installed without anything blocking access to the latch. Jun 6, 2023 at 1:13
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    that should not be a problem and is clearly not the case in this instance. a deadlatch can only be opened by operating the hande, that it why it is called a deadlatch.
    – Jasen
    Jun 7, 2023 at 9:03
  • The problem with disabling the bottom lock (as opposed to replacing it with a non-locking one) is that, instead of wasting your life locking and unlocking two locks, you waste your life explaining to everyone that it's not broken, it's not a problem, it's not unsafe, and we won't be home-invaded today because of it. Not worth it.
    – jay613
    Jun 7, 2023 at 13:12
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From a locksmith's perspective, you have a deadbolt lock and a spring latch. While the latter does have a locking knob and a deadlocking latch, it is considered primarily a convenience device; the deadbolt is your real security. This is a very standard combination.

Yes, there are places where the knobset alone is considered adequate security. If you live in such a place, you might want to use the deadbolt only at night, or only when the house is unoccupied, or at other times of increased concern. I know lots of folks who take that approach. But again from a locksmithing point of view, there are advantages to being just a bit more secure than your neighbors, so a burglar is encouraged to seek an easier target... so I do use the deadbolt. It's not that much inconvenience once you get used to it.

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The purpose is to give mental reassurance to the person inside that the door is secure and they are safe. Some individuals go to great lengths to get that sense of security.

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