A basic of master keying a standard pin tumbler cylinder is to add extra top pins to one or more pin stacks to create additional combinations. Each additional combination that is created will permit another key to work the lock. Simply put, if you add one top pin to one pin stack, it will create one new combination. If you add two, it will create two. If, however, you add one pin to one stack and one pin to a second stack, now there are twelve keys that will work this lock. This makes grand master keying possible, and it also makes it difficult to achieve securely.