Master Keys, What Are They

So what it is with the name ‘Master Key’. Without fail, almost daily, I will be asked if I have a master key for this or that lock. A Chubb Master Key, A Yale Master Key, A Master Key for postboxes (yes, seriously, I get asked this. The laws are clear on this, do not tamper with Her Majesties box!)

And yet any locksmith worth his salt will know that Master Keys exist, that they’re very useful, and also what they are and how they work is rather interesting.

So let’s shatter done myths and learn something. 

1 - What is a Master Key?

A master key is a single key that will work on all the locks in a particular group (and this is the important bit) IF the locks have been manufactured to have a master key. Here's an example: You own a hotel, there’s 20 rooms, each with their own unique lock and key. None of the keys will work on more than the lock it was cut for. And yet, should you lose your key, or lock it in your room, the landlord will take his ‘master key’ - a single key - and unlock your room. This key will work on all 20 locks.

How?

If you know how cylinder locks work you’ll know there’s a series of pins, each in groups of two, pushed into the plug (the bit you put the key into) by a spring, causing the plug to be obstructed, and not turn, maintaining a locked door. Inserting the correct key causes the split between the two pins of each pair to sit along the bit between the plug and the housing (called the Shearline), the peaks and troughs of the key lifting and dropping the pins a series of different heights to achieve this. When this split is along the shear line, the plug will turn, and turning the key will do this. Insert the wrong key and the splits between the pairs of pins will be lifted and dropped to the wrong heights, causing them to still obstruct the shear line and keep the lock, locked.

So how does a master key work?

It’s quite simple, and wonderfully logical. In a group of locks prepared for a master key, the pins are not in pairs, rather triplets. There are three pins to each stack. This means there’s two splits in each stack, meaning there’s 2 heights the pins can be lifted to to clear the shear line and allow the plug to turn, meaning there’s two possible keys for each lock. Of one series of splits is identical for each of the 20 locks in a group, one key (the master key) will work on them all! Ingenious!

As with seemingly everything in the world it gets more complicated though. As you can have more advanced master key systems. Assume you are the managing director of a company. You have three departments. Each department had two areas. Each area has a lock, and the staff member for that area has a key. Now the managers for each area need a master key for the two areas they manage. Now you, the managing director, need a master key that works on every lock in every area in every department. But the managers have master keys for the two areas in their department, but not the other departments. Confused? Of course you are. 

The principle remains the same. all the locks in the entire system have pins stacks in threes. The only thing that needs to be worked out, is the 'second' pinning, that being the other one to the grand master key - is different between the three areas in the middle section, so the three master keys don't work on the other locks along that level.

So - the people at the bottom have one key that only work on their lock. The middle management have master keys that work on both the doors in their area. The Managing Director has a Grand Master key that works on every lock in the system!

Now if that got your nogging jogging, bear in mind there's actually locksmiths alive who are working out far more complex master key systems, where you have a grand, grand, grand master key. Here...

A four-level master key system, how would you like to repin the locks in this system, or even work out the initial pinning? GMK - grand Master Key - MK - Master Key - SMK - Sub Master Key

People often ask me for a Master key for their lock group, and I hope now you can see why that's not an option - UNLESS they were happy to have all their locks replaced by ones with pin stacks in threes - a master key system!

So there you go, if you want a master key for a group of locks. Such as a hotel, or lockers, or garages, you’d need to buy a set of locks that offer the option of being ‘mastered’ - that is, having three pins in each stack, and therefore two possible keys; the single key for that lock, and the master key, for all locks. 

It's worth pointing out there are some locks that all use the same key and so that key could be called a 'Master key', but it's not really a master key system as there's not three pins and the option of other keys opening the lock. It's more just a lock that's always pinned the same. More a 'master lock' than a master key!