When lever locks wanted to be operable from both sides, the clever engineers at companies like Chubb thought carefully and insightfully about key symmetry and key differs. If you have a 5-lever mortice lock key and take a look at it you’ll see that it’s symmetric and that it has not 5 positions, not 10 positions but 7 positions (bittings they are called). (Unless you have an uncommon lock like a Securefast, Walsall, or one or two others.)

The first and third levers, counting outwards, are the same. This means that a reasonably short key can be used from both sides and still have enough lever variation to allow over a thousand different key possibilities. (If we call the levers A,B,A,D, and E, then a key that is EDABADE can operate the lock from both side while only sacrificing one lever’s worth of variation.)

When some miserable creature and their miserable company somewhere wanted to use a cylinder lock (like a “Yale”), rather than a lever lock, and also to be able to use the same key from both sides, they simply doubled the thing up and had a complete cylinder at either end. Well that’s not too bad — modern manufacturing can waste materials very efficiently; and with, for example, the Swedish profile cylinder system, it can be well designed and still quite secure. However, it requires two minutes more work to fit or change a “Swedish” cylinder, so this miserable creature (does anyone know who it was?) made a one-piece, double-ended cylinder — the euro-profile — the one that’s shaped a bit like an upside down exclamation mark. And when I say “solid”, it’s only just solid. Through truly stupid design, it’s actually very fragile in one very critical place and moderately fragile in others.

And because it’s one piece there are twenty different sizes to stock; and stock twice because you need to have them in brass and in nickel finishes.

And although this awful, wasteful device is the way it is in order to allow a key to work from both sides, if you accidentally leave a key in the inside half when you go out, you can’t actually use another key from the outside if the inside one has turned a little.

And — finally getting to what prompted this moan — if one of these things is fitted in a typical 45 mm door, their having duplicated the entire damned mechanism means that you can normally only get 5 pin tumblers each side unless you use a bigger cylinder than necessary, sticking out like handlebars, but allowing you to get a more secure 6 pin tumblers each side. So there’s no way for me to give one of yesterday’s customers what is needed.

So why don’t the States and continental Europe, and increasingly the UK, carry on fitting mortice deadlocks? Well it takes a bit more work as you have to chisel a nice neat, but quite large, hole in the door, if you’re fitting the lock later on in the life of the door. Or it’s more difficult and more expensive to manufacture if you’re mass producing doors. And perhaps, a lever lock key being longer and heavier, only the pockets and handbags of the UK of the previous century were up to carrying them.