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The Berlin key ("Berliner Schlüssel") was mostly used in tenements with multiple backyard houses with a common front gate that should be closed at certain times. These kind of arrangements were mostly used in Berlin, hence the name.

The other German names for the key describe different aspects of it: "Durchsteckschlüssel" translates to "push-through-key" and "Doppelschlüssel" translates to "double-key".

If you are more curious about the thing, there is a book by philosopher Bruno Latour on the Berlin Key where he describes the thing with the sociological situation that lead to it if I remember correctly (was a few years ago when I read it).

Edit: The basic idea is that in this kind of multi-party environment they wanted to make sure the door is always locked in the evening (when people return). To unlock the door you have to push the key through. Once you are inside you can get the key back, but only if you lock the door behind you. So once the door is locked, it stays locked, unless someone from the inside leaves it open on purpose. This way people cannot "forget" to leave it open.

http://www.bruno-latour.fr/sites/default/files/P-36-Berliner...

The building I live in in Berlin, 1910-ish construction with two more wings in the courtyard, everything accessible through the front gate - it has the same principle implemented with a timer and a button. I thought it's a bit odd, but made some sense after a while.

During the day, the front gate can be opened from the outside with just pushing a button that unlocks the door. At night (from 20 or 21 or so), the button stops working, you need the key. It can be opened at any time from the inside without keys. So it's more or less implementing the same protocol, but electro-mechanically, so there is no need for the trick with pulling the key inward.

I might be misunderstanding the old school original situation, but your modern setup does not enforce/encourage that the door is closed. Even an automatic closing mechanism could be blocked by something.

With the Berlin key if you want to get your key (ostensibly you do and certainly don't want someone else to take it) you have to close the door. Of course you can still leave it propped open but unlike in your modern setup you actually have some stake in making sure it is closed.

Sounds like a very German approach to following rules and influencing behaviour.

May I ask why you read a book on it? Just interested in locks/security or lock picking?

I’ve found reading general interest books are a great source of inspiration regardless of your speciality. So I like picking up random topics. For ex: a book on ants which had utility in simple autonomous network design.

Please contribute to the Wikipedia page, you seem to have some better information than is there currently.

It would be reverted immediately. Requires external sourcing.

The other names are already on the German wikipedia page: https://de.wikipedia.org/wiki/Durchsteckschl%C3%BCssel

So here a video is worth a million words :)

https://www.youtube.com/watch?v=4wE_6e3GPe4

Edit: I read the first paragraph twice and then to confirm my understanding I turned to YouTube.

They showed the key on video facepalm

I get the sentiment, but to mount this attack you’d:

* have to clone the key from fairly dark and jittery cell footage

* have to identify the location in a city of four+ million people

Once you did that you’d be able to access a commons area of a hotel?

I’m not convinced the risk is all that great :)

It also looks like a lever lock. Old lever locks tend to be pretty easy to defeat by means easier than cloning the key from an image.

Something vintage about video.

> Its name comes from the fact that it has two key blades …

Until recently, this sentence used to be “Its particularity comes from the fact …”:

https://en.wikipedia.org/w/index.php?title=Berlin_key&diff=9...

I agree it is really hard to parse. I think you're almost right. I'm pretty sure it's like this:

1. You can only remove the key if it is locked. 2. You can't lock it unless the door is closed.

That ensures that when you have the key then the door must be locked and closed.

The think about pushing the key through they door is just because otherwise you have no way to start inside the room, unlock and open the door, and then lock it behind you. Remember while the door is unlocked you can't remove the key. So if you just go outside and shut the door the key is now inside... and the only solution is to pull (or push) the key through to the door to the outside. Then you can lock the door and remove the key.