How Does a Master Key Work?

what is a master key

Mechanically, a master key works like any other key. The special ability of a master key comes from its ability to open different locks with a single key.

A master key is able to create a security system called a master key system, which allows only specific keys access while the master key holder has access to everything.

So what exactly is a master key and how does it work? We unlock those secrets next.

What is a Master Key?

A master key is any key that can open two or more uniquely keyed locks.

Despite this, master keyed locks are different from skeleton keys and keyed alike locks. That is a discussion for a future article, stay posted by signing up here on LockJudge.

How does a master key work?

A master key is unique as it allows its user to open two or more locks effortlessly. What’s the trick?

The trick is not in how the key itself is necessarily cut but rather how the cylinders of the locks themselves are pinned.

For a thorough explanation of how pin tumbler locks work, be sure to check out our article on “How do Lock Picks Work?”. In that piece, we explain the specific mechanics of a lock and how we are able to pick it open.

Recap of How a Lock and Key Work

Upon inserting a key into a lock, the pins in the cylinder of the lock are lifted by the cuts and grooves of the inserted key.

If the proper key is used, the key pins will push the driver pins clear of the shear line thus allowing the user to rotate the plug and open the lock.

Inserting the incorrect key will produce a different result.

The key will lift the drive pins once again, however, the key pins may be lifted short of the shear line, which would cause the driver pin to obstruct the shear line.

In another pin chamber, the driver pin may be lifted above the shear line such that the key pin itself obstructs the shear line of the plug.

That incorrect key will cause one or more of the pin chambers to be obstructed at the shear line and will not allow for the lock to be opened properly.

So imagine the inside of that lock.

Currently, there is only one combination of cuts of a key, the key biting, that will properly lift the driver and key pins to their appropriate positions along the shear line.

So How Does a Master Key Work?

In a standard pin tumbler lock, there are five (5) to six (6) pin chambers. Each with one set of a spring and two pins, the driver pin and the key pin.

Looking at two standard 5 pin chambered pin tumbler locks, each has its unique key biting. Looking closely, the keys are identical along the last four (4) notches. They are cut to the same height along this portion. The first notches do not align between the two keys.

Now in the first chamber of Lock No.1 in our example, we may insert an additional pin between the driver and key pin, this is called a master wafer. It is similar to a driver pin but normally smaller in height.

Master key

Master wafers come in different sizes to fill the gap between the master key and the unique lock key.

The master wafer in Lock No.1 creates a second combination of key biting that would allow a second unique key biting to open the lock.

This first pin chamber now has a new position where the key pin and the master wafer create a clearing at the shear line. The master wafer and the driver pin now also can possibly be positioned such that each is clear of the shear line as well.

Master Key Calculations

This video by Kokomolock does a great job of showing the logic and calculations required to make a master key.

The basics are as follows:

  • Identify the cut heights of each of the two keys
  • Write the cut heights for each key
  • Identify the shallowest cut heights between both keys
    These will be the bottom pins
  • Subtract the cut heights between the two keys
    These will represent the wafer pin sizes in each chamber

This master key calculation informs locksmiths on how to best cut the keys for the unique locks and the related master key.

For those curious, the formula of how many key combinations are possible is 2 to the exponent of the number of master wafers in the lock.

A lock with a master wafer in each of its six (6) pin chambers, can be calculated as 2 to the power of 6 and equals 64.

This means there are 64 different possible unique key bitings possible to open this one lock.

Or as I like to see it, 64 possible ways to pick the lock open.

With so many key combinations present, a locksmith can create different levels of access for each of the keys and their master keys.

This is called a master key system.

What is a Master Key System?

A master key system is a tiered triangle-like structure of the different master key levels of a security system, typically made for schools, apartment complexes or commercial office buildings. 

master key system

In a master key system, a group of keys allows specific keys to be able to only unlock their assigned locks and those that fall beneath their security level.

Change Keys

At the bottom of the master key system are the single keys that will open only one lock or door. These are called Change Keys and are at the bottom or are on the first level of the master key system.

Imagine a teacher with a key that only opens their classroom’s door. Their fellow teachers each have a key that opens only their own classroom’s door.

Master Keys

Above these individual locks and keys, is the Master key or MK.

Continuing with our school structure example, the school has a janitor tasked to clean the teachers’ rooms. Instead of carrying an individual key for each classroom, the door locks are pinned such that the janitor’s Master key will open any and all of the classrooms, but not the cafeteria or gymnasium.

The gymnasium and cafeteria fall under a different Master Key. Each of these rooms has its own individual keys as well as a specific Master Key which opens each.

Other rooms in the school belong to other categories, such as the administrative staff offices and the technology labs and/or automotive shop. Each room or access point has its own key while also being accessible by their groups’ Master Key.

Grand Master Key

Next, we go up another level in the master key system to the Grand Master key, also abbreviated as GM.

The Grand Master Key, in our example, opens all of the doors of this particular school. Despite some locks falling into different categories, the GMK is able to easily open each of them.

Great Grand Master Key

And to rule them all, we have the Great Grand Master key or GGM.

This key would belong to the head of the school district, thus granting them access and the ability to open all the locks that fall beneath them in the master key system.

In a master key system, security access can easily be maintained and does not require a massive ring of keys.

The Rings of Power from Lord of the Rings

The Rings in the Lord Of The Rings are actually a master key system, all be it one to rule the world but it the same as a master key system non-the-less.

Master key


A master key and system can provide reliable access restrictions within your business or building. The locksmith industry has also developed key tracking software in order to track keys, permissions and owners, as well as the various access each of the keys has.

Unique serial numbers on master keys may also prevent unauthorized copying unless authorized by the system owner.

As a master key system grows in-depth, the security of the individual locks falls. As we explained earlier, with the addition of master wafers, the ability to pick a lock in the system may increase.

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