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If you look at a mortice lock key from the side (https://tradelocks.co.uk/cylinder-mortice-borg-locks/key-blanks/6-lever-mortice-key-blank.html), it is symmetrical from tip-to-base. This is because the same key can be inserted into the same lock from either side of the door.

however, on some doors with simple cylinder locks (like this) https://www.easykeys.com/381_Yale_Lock_1-1600_Lock_Keys.aspx the key can also be inserted from either side of the door, yet it is not symmetrical.

how does this work?

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The mortise lock type for the key that you depict has a single locking mechanism that can be reached by the key inserted from either side of the door.

The cylinder lock you refer to is actually two separate cylinders that are keyed alike, that each have a simple rod at their back that activates the lock mechanism.

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  • Thanks - I had actually wondered if that was the case on account of the fact that the mortise lock keys are much longer – Andy May 10 '18 at 14:58
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    That's extra fun when you're in a key control system like Primus, and you have two cores at $90 each... Seriously one reason mortice locks went away is it's usually illegal to lock a door on the inside now. Too many burnt bodies have been found right next to such doors. In fact now, commercial doors must swing out with a push bar, because people inhaling smoke are too addled to handle more. – Harper May 10 '18 at 15:53
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    @Dan it's a fire safety thing, same as the electrical codes... – Harper May 10 '18 at 15:57
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    @bib: FWIW, I lived in an apartment with double cylinders where the key on the inside one actually broke off in the slot. Fortunately the door had glass panels that could have been broken out if needed, and I was able to turn and remove it with pliers anyway after spraying in lubricant, but it was a good lesson in why this is an idiotic configuration. – R.. May 10 '18 at 18:13
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    @Brad If you can, you should leave doors closed behind you when leave a burning building. This is so that air does not get in and feed the fire. In this scenario a smashed window is like an open door feeding the fire with bonus sharp bits! Doubly bad. That is assuming you can see which window to smash in the smoke. – DarcyThomas May 10 '18 at 21:29

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