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May 27, 2009 4:25 AM   Subscribe

What is the use of double locks? Not two separate lock, just the kind of lock that you can turn once (which locks the door) and then another time, in the same direction (which locks the door... more?).

If the door is only safely locked with two turns of the key, why would you offer the option to turn it only once? And if the door is just as safely locked with only one turn of they key, why offer the option for a second turn?

I've been thinking about his for some time. The only explanation I can come up with is that one turn is not enough to push the bolt far enough to really lock the door. But that would mean that all non-double locking doors are actually not safe? Surely that's not true.
posted by Skyanth to Grab Bag (13 answers total) 2 users marked this as a favorite
My grandmother's house had locks on the doors that required two turns. The first locked the "regular" lock (which could be opened from the inside just by turning the knob) and the second engaged a deadbolt (which required you to twist a little thingumy to unlock from the inside).
posted by Rock Steady at 5:00 AM on May 27, 2009

The only explanation I can come up with is that one turn is not enough to push the bolt far enough to really lock the door.

That's more or less your answer. To secure the door enough such that it can't just be kicked in, the bolt must engage the strong part of the door frame. If the frame isn't reinforced at its edge, then the bolt has to pass quite far to reach the strong part, thus requiring two or more turns of the key.
posted by randomstriker at 5:06 AM on May 27, 2009

The first turn locks the door which can be opened from both sides – outside with the key & inside with a latch.

The second turn engages a deadbolt that can only be opened from the outside with the key.

[Cue much hilarity as family member has to crawl out of a window having been locked in by someone who thought they were the last person to leave the house.]
posted by i_cola at 5:43 AM on May 27, 2009

i_cola has it. I have one of these locks, and wouldn't bother except that my front door has glass panels. Easiest thing in the world for a burglar to hold a coat or piece of cardboard against one of those panels, put the glass through with his elbow, and break in. But with the deadlock engaged his life is that much harder.

Also, if you have broken into a house and want to get away with the TV, computers, video, etc, you ideally want to walk out the door carrying this often cumbersome gear. It's that much harder if the doors and windows are locked, esp. if you climbed in through a broken window. It's tricky and possibly painful to climb out a broken window carrying cumbersome items.
posted by BrokenEnglish at 6:17 AM on May 27, 2009

Response by poster: Just want to say that I've never experienced locks where the first turn only locked it for "outside", so that's not the double lock I meant. Otherwise, if no-one is going to add to randomstriker's reply, it seems that indeed that would be it: single turn-locked-doors apparently have reinforcement closer to the door side (?)
posted by Skyanth at 6:18 AM on May 27, 2009

Can you please post a picture of the lock? I would like to see what you are describing. I had a lock once that turned more only from the inside. The extra movement made it not openable from the outside, even with a key.
posted by bensherman at 7:52 AM on May 27, 2009

Along with pictures, find out what actually happens as you turn the key with the door open and report back! As I see it, there are two possibilities: 1. the deadbolt progressively engages with each turn. 2. The first turn locks the door handle and the latch bolt (the one that's slanted on one side) and the second turn engages the deadbolt.
posted by zsazsa at 8:53 AM on May 27, 2009

I've seen locks like this before, and I always assumed that the bolt was simply longer than normal. Turning a key isn't like pressing a button -- you're physically moving the bolt with earn turn. So if the bolt is longer, it needs two turns to be fully seated. I've never seen anyone with a lock like that turn it once and then leave it.
posted by hayvac at 10:47 AM on May 27, 2009

Try locking the door while it is open. My understanding is after the first turn, it is still possible to use your finger to push the bolt back into its housing, whereas after the second turn you hear a click and it isn't possible to just push the bolt back. Something happens that makes that clicking sound and locks the bolt into the closed position.

With the door closed, if you only turn the key in the lock one time, it might be possible for someone on the outside to slide something into the jamb and scrape the bolt until it had come clear of the jamb so that you could just open the door. The second turn makes that impossible.
posted by nushustu at 10:50 AM on May 27, 2009

Also, I'm not calling anyone here a liar, but why in holy hell would you have a lock on your door that was only accessible from the outside? What if there was a fire? You would literally be locked in your own house. That makes zero sense.
posted by nushustu at 10:52 AM on May 27, 2009

nushustu: You're absolutely right. This is why double key deadbolts are banned in the fire safety code in a lot of places.
posted by qvtqht at 3:29 PM on May 27, 2009

why would you have a lock on your door that was only accessible from the outside?

As commented above, it allows you to leave the house and know that burglars can't take big items from your house using the door.

Land Rover's have/had this feature on their vehicles. I think they called it superlock or something. The idea being that you could lock the car door such that a thief couldn't open it even if they broke the window. As long as your packages inside are bigger than the window frame, then the thief can't take them.
posted by jsonic at 3:47 PM on May 27, 2009

why would you offer the option to turn it only once?

I used to work at a place that had a deadbolt with a long, thick bolt. Instead of sliding out of the doorframe, the bolt pivoted up into the doorjamb. It was a large enough bolt that there would not have been room for it to slide into the door next to the big glass panel.

The key had to be turned more than once to lock it, and could not be removed unless the door was either locked or unlocked. When the door locked, the bolt slid rather loudly into place.
posted by yohko at 4:36 PM on May 27, 2009

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