Many customers don’t realize that their rimlatch (their “Yale”) lock might be of the double-locking type.
If there’s a keyway in the inside handle, what’s it for? Well, first of all it might not work. If you had the outside cylinder (or barrel) changed at some point because of lost or otherwise compromized keys, chances are that you didn’t get the inside cylinder re-keyed at the same time, so it needs a different, probably lost, key.
Its purpose, though, is not what you’d think. It locks the inside handle/knob.
Why would you lock the inside handle? Because if someone has broken into your property at the back, they would probably prefer to carry your plasma screen TV out of the front door. But if you’ve locked the handle they can’t.
Your rimlatch might still be double locking even if there’s no keyway in the inside handle. It’s quite often the case that you can lock the bolt and the inside handle/knob if you turn your key full circle in the opposite direction to the normal, unlocking direction.
Now, the main reason for this post if you’ve the latter variety of double-locking rimlatch, is to warn you that you might lock someone in one day. It goes like this: you never realized that the latch was double locking; you left as usual pulling the door shut behind you; you realized you’d forgotten your umbrella; you unlatch the door leaving your key in the lock as you grab your umbrella; then you pull the door shut again and turn the key. Now anyone left inside can’t get out. Even if they have their outside key they can’t unlock the inside handle. The only thing they can do is call to a hopefully honest passerby and pass them their key through the letter slot.
By the way, if your key takes three-quarters of a turn to open the door, rather than the more normal quarter turn, then it’s possibly a double-locking lock that’s had its double-locking disabled.
(In the UK, you may well have a mortice lever deadlock as well as a latch. Once that’s locked then your door has also been secured in both directions. For some reason mortice deadlocks never gained popularity in the States or in continental Europe, where break-ins are easier — although not necessarily more common — Brits seemingly being a villainous lot.)
What do we mean by locking the bolt? A latchbolt latches: you don’t need to use any key in order to make the door shut behind you. This happens because the latch is sprung and the closing face of the latchbolt is angled whereas the opening face of the latchbolt is flat. This convenience is also a security hazard because a burglar might be able to get something to push against the sloping face and unlatch the lock. So that’s why we might want to lock the latchbolt.
(If a lock bolt is unsprung and a key required both to shut it and to open it, it’s what we call a deadbolt — it’s not live, i.e. sprung.)
Some expensive locks — like an Ingersoll — automatically deadlock as they latch shut.