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Coming from the United States down to South America, in Peru, Argentina, and Chile, I have run into a number of home door locks that I don't understand how they're intended to work.

  1. If the door is unlocked, you can exit, but the door locks behind you, needing a key to reenter. I suppose this makes sense and you'd just want to make sure you always leave with the key.

  2. You can insert the key into the keyhole and rotate one direction, quarter turn to open the door, or the other direction one rotation and it engages the deadbolt. Problem is, if someone is still inside, you've locked them in, as there's a keyhole but no turn piece on the inside. That means anyone still inside needs a key to leave.

  3. From the outside, if you rotate the key an additional rotation for a total of 2, I believe it pushes the deadbolt further out, and I only have one key to test this, but I'm only able to get to this state from the outside, from the inside, it only allows the single rotation. I believe this to mean that I could potentially be locked inside with the key but I wouldn't be able to exit? Is this correct?

I'm able to understand the first way the lock works, but the second two baffle me. If there's a fire and the lock is in state 2, I'll either need to break the door down or use the key to leave? If the lock is in state 3, even if I have the key, I wouldn't be able to exit unless I slide it under the door to someone and have them open it for me? My overall question is what are the use cases for potentially dangerous lock states?

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    The 3rd one is weird, but the second one is quite common in other places, not just South America. – jcaron Jan 16 at 21:19
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    I've heard of state 2, but not state 3. 2 is not unusual in the US; it's for doors with a window that could be broken. A key could be hung near the door, but far enough from the door that it can't be reached through the broken window. Not as safe as a lock that can be opened by hand from the inside, but some people may consider it reasonable. – Gerard Ashton Jan 16 at 21:24
  • @GerardAshton It all depends on what you are protecting. A deadbolt that does not need a key stops someone breaking in, but allows easy egress for those inside. So safer for people. A deadbolt that requires a key on both sides stops someone from breaking in and waltzing out the door with all your stuff. So safer for your possesions when you are not home. I grew up with the latter and find the former strange. – Peter M Jan 16 at 23:20
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    Living in South America, I always thought it was a terrible idea! Not being able to exit in case of emergency is particularly worrisome and plenty of rooms do not even have a window to use in such case. – Itai Jan 17 at 4:11
  • These locks are common in Central Europe and I hate them too. It shouldn't be possible to lock yourself out while leaving the key, especially in safe cities. – JonathanReez Jan 17 at 8:13
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Lock type (3) sounds like a 'deadlocking nightlatch'. Common in Britain, where I live, the one described here costs 20 GB pounds.

From the maker's website

enter image description here

Advantages: You can lock or disable the handle from working inside, this is useful if nobody is in the property. If someone smashes the glass (if it has glass) they can’t unlock the door. It also prevents the latch from being slipped. Disadvantages: If someone is in the property you could lock them inside. Lock Standard for Deadlocking Night Latch: Deadlocking night latches are not approved to British Standard (BS).

Security Note: A deadlocking night latch (non BS 3621) should always be used with a BS 3621 5 lever mortice deadlock for security.

How the lock is operated: Again a deadlocking night latch acts the same as a basic latch but with the extra feature of locking the latch with a key.

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  • This answers the question I was most interested. The purpose for having both the 2nd and 3rd state. I played around some more with the lock at the place I'm currently staying at in Peru. Indeed, I was able to get the 2nd rotation from inside by jiggling the key a bit. However I had a similar issue last year in Argentina, where the key would easily unlock from the 2nd state, but if someone rotated it to the 3rd state, no matter how much I tried, I couldn't get it to unlock. – Travis Jan 17 at 15:05
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That's also how most locks work in Spain, at least for points 1 and 2.

The first mode ensures that the default state is a locked door; you can't foget to lock it after you, as soon as you pull it closed it will engage (incidentally, many locks are trivial to open in this state without a key so not very safe anyway).

The second case is easily solvable as most people I know leave a key inserted at all times. The last one to leave the place takes the key from the door and closes behind them.

The third state is a bit strange, I've never seen a lock that doesn't fully engage from both sides; but I bet you can unlock it from the inside even in that state.

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