Occasionally, someone will ask that the locks they use in their institution, organisation etc have 'mastered' locks. This means that although all the individual locks have individual keys, there;s also a 'Master' key, which will open them all. A good example would be in a Gymnasium locker room. The idea being should someone lose their key, the staff can still give them access to their locker, so they're not leaving the swimming pool in the nude!
To make this work, the locks have 3 pins rather than the usual 2. The 'actual' key works by aligning the first split in this set, the master key aligns the second split. However, with 3 pins and 2 splits, the possible combinations are a lot less, and so security is slightly compromised. Especially since 'Mastered' locks tend to use very predictable and common pinning patterns. Over the years, lots of these predictable and common pinnings have been recorded, and those patterns are what our master keys are based on. Making them very effective 'try-out' keys when working in the field.
The other Master Keys such as our Fire Brigade keys work because the fire service has maintained the same pinnings to mean different forces up and down the country can ensure access to the fire service locks.