My early recollections of developing master key systems included the use of a master key system printed in the beginning pages of the leather-bound Reed Code Book. As the same cuts were used over and over again, notes written in the margins showed what keyways had already been used. If a second master key system was required, we reversed the cuts tip to bow and used the same Reed master key system one more time.
Several enterprising locksmiths later published books of master key systems. At least then there were more code systems to choose from. The problem with these one-size-fits-all master key systems was that it was just a list of change keys. When a customer required floor master keys or if all change keys had to operate a vestibule door, these key systems were not very helpful.
Hollis Devine is often credited with developing a master key system in the 1960s which is known under various names such as The Quadrant System or Hold & Vary. By coincidence, popular key systems such as Sargent, Yale and Schlage used a 10 depth increment system and a two-step progression. This provided one master key cut and four remaining cuts which could be used for the change keys. A setup chart for hold & vary systems contains the top master key cuts and a key bitting array listing of the change key cut choices which can be used in each space on the change keys.
The basic rule for a hold & vary key system is that all change key cuts are taken from the key bitting array list and all lower level master keys are developed by using a combination of change key cuts and top master key cuts. When rules of hold & vary are followed, the lock cylinders are not worn or mutilated, correct pin lengths are used and keys are cut to manufacturer depth/space tolerances, hold & vary is a foolproof system to implement.
Mr. Devine developed a single typewriter symbol for each possible individual top master key cut and each cut in the key bitting array and showed by symbols how each top master key, lower level masterkey and all change keys could be cut. Armed with the symbol charts developed by Mr. Devine, a locksmith could set up an array of numbers and plug the real numbers in place of the Devine symbols and build a master key system with new key cuts for every job.
While commercial locks such Schlage, Sargent and Yale fit easily into the quadrant system formula, companies such as Kwikset, Dominion and others either used a single step system or had less than 10 depth increments with which to produce a true quadrant key system. Fortunately the same Devine formulas still apply, the only result is that more or less change keys are available as dictated by the amount of cuts in the key bitting array.
Hold & Vary quadrant systems provide a set amount of change keys and master keys. A five pin system automatically develops 1024 theoretical change keys, a 6-pin system develops 4096 theoretical change keys and a 7-pin system develops 16384 theoretical change keys. Depending on the allowable maximum adjacent cut (MAC), usable change keys may be approximately 10 percent less than theoretical change key amounts.
A quadrant system automatically provides pages of 64 codes per page. A 5-pin system contains 16 pages, a 6-pin system contains 64 pages and a 7-pin system contains 128 pages. There is a master key which operates each page, a master key which operates each four pages plus vertical and horizontal master keys for groups of 16 change keys on each page. Master keys which operate larger amounts of pages are available for 6-pin and 7- pin systems.
Computerized master key programs took up where Mr. Devine left off, and locksmiths of today have easier, more accurate ways to generate master key programs.
During the mid 1980s I became employed by Abloy Security Locks and was sent to Finland to learn about a different master key system called rotating constant. Rotating constant key systems begin development just like hold & vary. A top master key is chosen and a key bitting array of available change key cuts are developed for each space on the keys.
The basic rule for rotating constant is that every change key cut in the system will contain at least one cut from the top master key. In a 5-pin key system, the possible four-part rotation sequences are 1234, 1235, 1245, 1345, 2345. 6-pin and 7-pin systems have an even greater amount of rotation sequences.
Change key cuts in a rotating constant system are called differs. As example, while key bitting array cuts are rotated in positions 1234, the fifth space on each differ cut will be given the same cut as the top master key. If a rotating constant system is generated by using four cuts in each space in the key bitting array, the total theoretical differs for each rotation group would be 256 differs and five groups would generate 1280 theoretical total differs.
While domestic lock companies usually use ten depth increments or less, height dimensions of key blades on some foreign lock companies are larger. As an example, one European lock company uses 23 different depth increments. Instead of having only four key bitting array cut choices per space, this company can at least double the amount of available cut choices per space. However, this same lock company uses a cutter angle of 110 degrees which then allows a smaller maximum adjacent cut (MAC) as compared with companies using a hold & vary key system. The result is that a large amount of the 23 different depths cannot be used together and the benefit of having 23 depth increments is less significant.
A rotating constant key system can develop more differs when compared to the amount of change keys available in a hold & vary system. However, each differ contains at least one master key cut. Most master key systems will require the use of lower level master keys such as for floor masters or suite masters. Development of lower level master keys in a rotating constant system is a challenge since some different key cuts will automatically become unwanted lower level master keys. As more lower level master keys are required, differ keys which will cause cross-keying must be eliminated from the system. There is a direct correlation in a rotating constant system between any increase in the amount of lower level master keys required and the decreasing amount of available differ key cuts.
The solution for companies which user rotating cut key systems is to offer sectional keyways. As example, note the Zeiss-Ikon section in a Kaba-Ilco key catalog. There are dozens of sectional keyways available. Each floor master key can then use a different keyway and the same original differ key cuts can be used again and again with each different keyway. A thinner master key blank can be used which fits into all the various keyways in the key system.
There are very few instances when a locksmith dealer will be called to rekey or upgrade building security by just furnishing a new top master key and an amount of change keys. Almost always there will be some additional requirements such as for floor master keys, a vestibule key system or suite master keying. A hold & vary system allows a locksmith to either rekey the existing standard lock cylinders or to furnish new high security lock cylinders which are in stock while using only one keyway.
A locksmith called to a rekey or upgrade building security when a rotating constant key system exists is faced with different circumstances. This locksmith must be prepared to write a new rotating constant key system while having the key blanks available for any special existing keyways. The alternative is to place a factory order and have the customer wait while the new high security lock cylinders with the different keyways needed for developing lower level master keys are delivered.
Sargent has been using a 78 degree cutter for many years. Most other domestic lock companies use cutter angles of time-prove 86 to 90 degrees. The solution would be for European lock companies to change their cutter angle from 110 degrees to 90 degrees. This would raise the maximum adjacent cut (MAC) number and permit full use of a hold & vary key system while not requiring 23 depths to do the job.
