Master Keys are used by many people and businesses but not many understand how they actually work. How is one key opening two doors but another key can only open one of those doors? After Reading this you should have a better understanding on how a master key system works.

Keys in a Master System

In order of least access to most access:

SKD –  This denotes a Single Keyed Different cylinder. Sometimes you want a door that is not keyed to your master-key system for security reasons. No key except the SKD will work this door example might be a HR office.

Change Key – This key is also referred to as a sub-master key. It will open one lock and only locks that are exactly the same. The lock that the change key opens can also open with the use of the master key, and any key above that rank.

Master Key – The Master Key sometimes referred to “MK” this key will work all change keys that fall under it in the system.

Grand Master Key – A grand master key is used to access multiple master key systems. This key will open every master system under it, and the subsequent change keys under those systems. In the trade, a master key may be shortened to ‘GMK’.

Great Grand Master Key – The great grand master key will open all the grand master key systems under it, the master key systems under those, and the change keys under those. This can continue further with a grand grand master key and on and on.

How Master Keys Work

master key diag

Basic pin tumbler locks open by raising the key pins so that the driver pin is above the sheer line and the key pin is below. The key pins will be different sizes with accordance to the cuts of the Change keys and Master Keys, all the driver pins will be the same size.  When you master-key the lock you add additional master pins between the driver pin and the key pin. This allows lock to work with two different size key pins. For example lets assume the first cut has a key pin depth of 3 with a number 2 master pin. A key that is cut as as a 3 will work as well as a key cut to 5, note that a key cut as 2 would not work because the key pin is on the bottom and the 2 master is above.

Key Take A Ways

  • Master key system is a system that allows two or more keys to open one lock.
  • Master key systems often use pin tumbler locks.
  • Pin tumbler master locks use a master wafer/pin in between the key pin and the driver pin.


Master Keys serve two purposes that make your life more convenient. Most larger business don’t want to carry around several keys to gain access to every room. They also might want to restrict  access to certain areas of their building to their employees.

Security Vulnerabilities

These conveniences do have a draw backs though, they also will lower your security. Master keyed locks are more vulnerable to a few types of attacks:


Adding the master wafer/pins to your lock creates a secondary shear line. Just as it adds the possibility for a second key to work in the same lock it also adds another point for you to pick the lock. This alone does not change much as any professional can single pin pick most pin tumbler locks with or without the master pins. If you have several chambers with master pins it makes the lock more susceptible to raking as the pick method which can be done by most amateur lock pickers. When deciding to master key or not you have to access the risk vs the added security. How common are picking attacks in your area? How much will key control and personal convenience add to your business?


Exploratory keys are used to decode master keyed systems and only requires your change key and a lock that it opens. This process takes time and is done by changing one cut at a time starting at zero and progressing the the deepest possible depth. If the cylinder turns you have discovered a master depth. If it only turns at the depth of the change key you know a master pin does not exist. Due this process to each chamber one at time to decode the master key.


If you are looking to have your locks master keyed or another commercial locksmith service in St. George, Ut. give B&E Locksmith a call at  435-414-9753. We hope to hear from you soon.