If you memorize it, you'll always know which notes are flat in any of the given keys. Bb - one flat (Bb). Eb - two flats (Bb and Eb). Ab - three flats (Bb, Eb, Ab), and so on.
If you go the other direction and add sharps, it's the circle of fifth.
Mike - Militant Left Hander
See profile for list of TB clubs I'm a member of