It could be that the double twist engages the dead bolt function. I'm not sure but that's the only reason I can think of.

AFAICT, it's a deadbolt either way...

Are you turning the key or the knob on the inside? If it's the key, does that lock the knob on the inside?

If yes, then that's your answer. If no, then I dunno.

Warbaby, I don't understand your question. There is a door handle and a separate deadbolt. On the outside the deadbolt turns with a key, on the inside with a knob (not the door handle).

My locks do this (both of them). More bolt, harder to kick in the door, no? Plus, it's probably harder to pick the lock if you have to make 2 full turns.

I've had locks like this. I always figured it would require smashing out more of the doorframe to break in when the bolt was extended farther.

In both cases the door will go before the door frame, unless it's a solid block of wood. I don't think that turning it twice really offers more security, but you could double-check the measurement and see how much of the frame (vs. just the trim) is engaged by the bolt for a double turn.

Thanks for all the responses. In conclusion, the extra turn *might* make the lock more secure but I think there are other ways of breaking in that are easier than pushing the bolt through the frame, so it's hard to see that it matters.

If it is two complete 360 degree rotations, and on each rotation the tumblers and set pins of the lock do indeed pass the shear line twice, it would make it somewhat harder to pick. But if it was pick-able, you'd just have to pick it twice.

But I've never heard of that as an intended security feature. There are much better ways to make a lock harder to pick, like rounded, beveled or mushroomed pins and chamfered pin guides, tighter tolerances with less slop in the cylinder and all around, more restricted or convoluted key ways. But I'm a total noob when it comes to locksmithing, so maybe there is something about the double rotation.

However, I have seen those locks. They're not that uncommon.

2nd try: does turning the key twice (and making the bolt stick out farther) prevent the knob on the inside from being turned?

In my experience with doors being kicked in or forced, the doorframe has always been the weaker part that breaks, even on old, thin, stile and panel doors. This is usually because the bolt didn't engage very far in the strike plate and the strike plate screws mostly held onto into the trim and did not grip enough wood in the door frame. The tighter a door fits, the stronger the hinges and bolts are. Loose door: easy to force.

In rapid forced entry, they go after the hinge side. This is usually done with a shotgun loaded with solid shot or special powdered-lead "doorbuster" rounds.

it's not enough to just turn once, really...you want it jammed in there. (but i wouldn't worry about it too much, especially if you're in an apt. bldg)

I'm pretty sure it's illegal for the knob on the inside to be locked from turning. Fire regulations and all that: you gotta be able to flee when the house burns down.

Warbaby: no. You can turn the knob on the inside twice too.