A Key Series carries a range of different, normally sequential numbers called 'differs'.
Often, the number is engraved on the face of the lock and each differ number has its own unique key.

All locks are opened by the same key.

A Master Key will open all the locks although each lock will have its own unique key.

Sometimes referred to as a Fitter Key will remove the lock barrel. This facilitates the easy replacement of cylinders without having to replace entire locks.