Thoughts On Master Keying

Oct. 1, 2010
With the introduction of electronic locks many years ago, some locksmiths predicted that by this time all metal keys would be extinct. Nothing could be further from the truth.

Specifically in the automotive industry we have seen continuing progress in the reduction and elimination of mechanical keyed cylinders. Fewer and fewer cars each year have keyed cylinders in the trunk, doors or glove box. Even the keyed ignition switch is disappearing.

In the commercial and residential world there are multiple advantages to electronic security, but common pin tumbler locks and master keying are still widely used. In addition to electronics, various high security lock manufacturers offer second and third level locking components that further reduce the loss of security, but in many instances it is a matter of expense that determines the continued use of simple pin tumbler locks in a system.

Master keying is still alive and well. For the purposes of this article the term ‘master key’ refers to the top or highest level in the system.

In standard mechanical master keying a primary fundamental is that master keying is a controlled loss of security. Each pin stack consists of a spring, top pin or driver and bottom pin. Master pins are added to this stack to create additional shear lines. (Illustration #1)

When the number of shear lines increases, security decreases. Each additional shear line added further reduces the security level. By keying a cylinder to fit two random keys, you can create a cylinder with dozens of shear lines. In addition to the two intended keys, dozens of other cut combinations may work that lock, resulting in an uncontrolled loss of security. Either by picking, key manipulation or simple key insertion, unauthorized access may be granted. (Illustration #2)

When you are designing, implementing or servicing master keyed locks, it is essential to remember the limitations of a master key system. Following are some things to consider.

System Size

A fully progressed two-step 6-pin system will yield a maximum of 4096 cuts while a fully progressed 5-pin system will result in a maximum array of 1024 cut combinations. Not all of the cuts may be usable. For example, although easily cut, a key with cuts of 1-1-1-1-1-1 isn’t a good key to issue. A key with cuts of 7-3-8-9-0-9 isn’t possible in most systems because of the Maximum Adjacent Cut Specification (MACS) rule, also known as the maximum allowable cut. A nine cut next to a zero will cut off too much of the adjacent material of the key blank to allow a zero pin to seat properly. Many manufacturers specify a MACS of 7 values between adjacent cuts. Know the limitations of the system you are using.

The system size you need is determined by how many individual keys your customer needs now and by allowing for future growth of the system. When a system is initially written, the size of the system should be adequate to fit the customer’s needs today and allow for future re-keying and adding new locks into the system. Ask your customers how often tenants move in and out; see if they are planning on adding new doors or buildings, etc.

You wouldn’t want to write a fully progressed 4096 cut system for a 30-unit office building. A small 64-cut system might be adequate, or if there is a lot of re-keying expected, you might go to the next level with a 256 cut system.


Some systems today are still hand written on the job. In a hand written system, you may be a couple of pages into it before you discover a large amount of undesirable key cuts. Even in a computerized system the progression sequence must considered. A computerized system will allow you to experiment by changing the number and position of constants carried and the order of progression. By a simple keystroke, you can write and evaluate the feasibility of a new system or rewrite it easily before putting it into use.

If you are initiating a new system, be sure to examine the cuts of the old system. What kind of patterns do you see? Using higher cuts where there were low cuts helps assure that no key from the old system will work any lock in the new system. In a partially progressed system, alternate the chambers carrying constant values. If the old keys had a constant in the first chamber, use the fifth or sixth chamber in the new one for constants.

In a fully progressed system each chamber uses all of the cuts available in that manufacturer’s system. In most common 10-depth, two-step systems the cuts range from a shallowest cut of 0 to a deepest cut of 9 for a total of 10 available depths. (Illustration #3)

The two-step means that for each given cut of the master key, every other available cut is used in the progression of the system. If a MK cut is an even number in the first chamber, all cuts used in the first chamber will be even. In a chamber with odd cuts in the MK all individual cuts used in that chamber must also be odd. Since we will have five available cuts in each (even or odd) chamber and one is used in the master key, we have a progression value of 4. A mathematical formula dictates that 4 is the prime number in progression.

Determining The KBA

The Key Bit Array (KBA) is also known as the Progression Chart. The first step is to determine the cuts of the master key. After the cuts of the master key are chosen, each chamber can be progressed out. In a partially progressed system, some chambers are progressed and some remain constant. Carrying constants reduces the number of available cuts. (Illustration #4)

Since each chamber offers four possible cuts, the number of cuts in each chamber is multiplied by the number of cuts in the next chamber. The more chambers progressed, the more cuts available. Progressing three chambers will result in 64 cuts (4x4x4) and leave three chambers to be used as constants. In common use, this is considered a one page system with 4 columns of 16 cuts each. Expanding progression to four chambers will give you four pages of 64 cuts each with a total of 256 cuts (4x4x4x4). Remember the exponential formula, the number of shear lines in a chamber times the number in the next chamber.

Therefore a fully progressed 10-depth, two-step system will give you a maximum of 4096 cuts because all chambers are progressed (4x4x4x4x4x4) or 4 to the sixth power. (Illustration #5)

Common Areas

One of the biggest problems faced is that of the common area. Whenever a building is master keyed, the issue of shared entry doors, bathrooms, etc. is raised.

People all have a key to their own doors but don’t want to carry an extra key for the common areas. Loading a cylinder with a low bottom pin and a stack of master wafers or pins can cause lock problems with jammed pins or keys working when they shouldn’t. A 6-pin lock with that setup can actually have thousands of shear lines! Almost any key inserted will open the lock.

Security is tighter when a totally separate key is issued for those doors. Explain to your customer the risks involved and the loss of security.

When it is necessary, here are a couple of ideas to minimize problems and keep security as tight as possible. By keying only the first and last chambers of the lock in a system carrying constants in those chambers, a key with specific cuts will be required to gain entry.

In a 5-pin system, use a 6-pin housing and utilize the sixth chamber. Fully insert a 5-pin key from the system and insert the proper length pin in the sixth chamber to achieve a shear line. The point of the pin will actually rest on the slope of the tip of the key. This will ensure that any key used from the system must be fully inserted into the lock before it will operate.

Decoding An Existing System

When you take over a new building from a customer, there is usually an existing MK system in place. If the customer doesn’t have a copy of the charts, you don’t have the KBA or progression information. You also don’t have the number of cuts used, unused or what key was issued for what door.

Aside from gaining access to the information from the previous locksmith servicing this account or a total re-key, you may be able to determine a lot of the missing information from what is at hand. This is not a slam-dunk guarantee, but it can certainly help. The problem is that it takes time and effort.

First, decode the master key information and write it down. Then start decoding existing keys in the system, writing down the information. You should soon see some mathematical patterns develop. If the MK and all of the keys in the system have a 5 in the first position, that may be a clue that it was used as a constant throughout the system. If the MK has a different cut value, it may indicate it is part of a rotating constant or the system was progressed to handle floors, departments or areas that group keys in sets. Example: All keys on the third floor have common characteristics or all keys in the marketing department are from the same page or group.

This sort of detective work will provide valuable information that can help you determine the feasibility of expanding or continuing with this system. Careful examination of known cut combinations will reveal which chambers were progressed and in which order.

Incidental Master Keys

Without some clues, you are keying blindly and a new individual office key you generate may have been previously issued as a sub-master or incidental master.

In a properly written system if you pin your lock to the top master key and the lowest individual key, any incidental master keys will work automatically. An incidental master is a generic term for shear lines created by the mathematics of the system.

The mathematical progression will cause each page of a MK system to mirror the other pages with one chamber being different. A fully progressed system with no constants will produce 64 pages of 64 cuts per page, a total of 4096 cuts. Remembering the prime number of 4, each page will have 4 columns with each column containing 4 groups of 4 cuts each.

In addition to the top master key or grand master, each page will contain a page master that fits all 64 cuts on that page, a column master that will fit a vertical column of 16 cuts, a horizontal master that will fit 16 cuts grouped across the page and a group master for each group of four cuts in sequence. Again that prime number of 4 surfaces.

You can’t eliminate these mathematical shear lines that allow these sub-masters to operate. You need to be aware of them and use them to your advantage. Like spaghetti sauce, it’s in there whether you want it or not.

These groupings allow you to issue keys for areas that will provide a sub-master to fit only the locks in that group of 4, 16 or 64. If you need 9 individual door keys and a sub-master, choose a group of 16 and use the built-in mathematical values. Utilize these to create floor masters, department masters, etc.


A properly written and executed system will allow you to assign small or large groups of cuts to your project. Simple keying actions will result in a flexible MK system offering the maximum amount of security to your customer.

Remember that random keying to various cuts is dangerous and destroys any sense of a controlled loss of security. Control the loss with a well thought out system to meet your customer’s needs.