Apple Build Numbers


Can anyone explain to me the build numbering system Apple uses for OS X?

I'm currently running 7M34 (more commonly known as 10.3.5)

There is a list of build numbers here that confirm that the first number is the major release):

But when does the following letter get increased? Even the final number doesn't seem likely to go up after after every build judging by how close some of the numbers are between successive minor releases.

Any readers know, or at least care to speculate?

UPDATE (2004-11-02): The Tiger developer preview that was just announced is 8A294. So they are still on the 'A' series of 10.4. I wonder when they start 'B'.