If it’s a Yale latch lock and it’s on the right-hand side of the door, then a loose snib might just have fallen down when the door was slammed shut. (The snib is the tiny button or lever that holds the bolt open or closed. A dropping snib on an ERA on the other-hand, is only going to lock you out if it’s on the left-hand side of the door. (These handings are as if you’re standing outside.)

Another possiblity for any lock with a snib is that there’s a huge gap between the door and the frame; and you left the lock snibbed yet managed to slam it shut.

One, more disturbing possibility you should bear in mind however, is that if a thief has broken in through a window or throught the back door, (s)he will usually snib the front door’s latch so as to get some warning if you return.

So, enter the premises cautiously. Send the dog in first.

If you’re choosing a latch lock, some can be snibbed open or closed, but some can only be snibbed open (also known as a hold-back).

A latch lock is the kind that can slam shut behind you, locking you out. Usually when you’ve decided to nip quickly out onto the doorstep in your dressing gown in order to get the milk delivery.

The snib is the little button that locks the latch bolt in the open position, and sometimes in the closed position as well. There are at least two reasons for checking that the snib isn’t loose.

Firstly if you have a dog (or cat?) that has the habit of jumping up and pawing at the locks, then Fido just might manage to activate a loose snib and lock you out.

This has now happened twice to one of my customers. They didn’t believe me the first time and wouldn’t let me change the lock. Now they are believers.

Secondly, if a snib has become loose and the lock is a “nightlatch” style lock on the “dangerous” side of the door (for a Yale 77, 84, 85, 88 or 89, for example, the “dangerous” side is the left as you look from the inside) then slamming the door could cause the snib to drop and lock you out — if down is the locking direction. (When mounted on the other side of the door a dropping snib is dropping towards the open and safe position.)

Snib or holdback are the correct terms for the part of a lock that stops a door latching behind you as you pop out to the dustbin in your dressing gown. It’s often incorrectly called the latch; latch, however, is what a door is doing if it locks as it closes, without your having to take any action other than close the door.

On locks that don’t have much sophistication, as well as holding the latchbolt open, the snib can also lock the latchbolt closed even against the correct key.

A dropped snib is when the snib gets loose, and when the lock is fitted to the unlucky side of the door, and when as you slam the door and it latches shut, the snib falls down and locks you out.

So, which way would your snib be going if it fell downwards? Would it be falling towards inactive? Nice green tick! Or falling towards active? Nasty red cross! If, for example, you had a Yale rimlatch lock on the right-hand side of your door (looking at the door from the outside), then I’m afraid it doesn’t fail safe, it fails the vexatious way.

If you have a simple latch lock and it’s on troublesome side of the door, keep an eye out for the snib getting loose and floppy.

If you like finding things to worry about, I have one more for you. Is there a big gap between the door and the frame? It has been known for a latch lock to be snibbed to the locked state while the door is actually open, and then for the door to slam shut. The gap is big enough that the bevelled latch bolt can enter the bolthole, but it can’t then be got out again.