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The London Underground is famous for all kinds of reasons. It was the world’s first underground railway network; its tunnels extend staggering distances; the distinctive topographical Tube map… Read all about these things in The Guardian here.

One thing that is usually overlooked is its locking system.

Winkhaus logo


Near enough every lock on the Tube is made by Winkhaus. Legend has it that the entire network is part of one single master suite, which would most likely make it the largest master suite in the world.

Do we believe this? Not really. It’s too good to be true! Winkhaus is, I’m sure, capable of creating a suite that big, but why bother?

Having a grand master key that can open every single door on the network is, more than anything else, asking for trouble. Imagine if that key got lost? Any old urchin who finds it could cause all kinds of havoc!

Still, the idea is interesting. Even if there is no grand master, is a suite of that size possible?

What is a master suite?

A suite of locks is a set of locks that each have their own key, but that also have a master key that can open all locks. This setup is common in hotels, for example, where a holidaymaker uses their key to access their own room, while the cleaning staff have just one key they use to access every single room.

You can get very complex suites, too, with sub-suites. Imagine you’re living in a gigantic country estate. You can have a separate suite in the East Wing, West Wing, the servants’ out-house and anywhere else you see fit.

You can supply your staff with a master key that will open all doors in the West Wing, but that won’t open anything else, or you can have a grand master key that opens everything.

From here, things can get even more complex. Some keys might open all doors on sub-suites 1, 2 and 3, but not sub-suite 4.

But the Tube has over 250 stations, which would mean over 250 sub-suites — not to mention that there could even be a sub-suite for every individual line. And each station sub-suite might have up to 100 locks on it. That’s 25 000 locks all on one gigantic master suite!

Is that even possible?

The answer is yes! It all depends on how good your maths is, really. Most of the fine details about master suites are worked out with a computer. The software calculates thousands of ways you can put a suite together, drawing up permutation upon permutation.

It creates subtle variations in the moving parts within barrels, for example, which allow one key to work, but another not to work.

I’ll bet that, in the olden days, all this was done by mathematicians. Now, with the right software, even you could make a master suite.

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