Use your basic Movable Do Disc with the Absolute Pitch Attachment (APA) combined with the larger APA to transpose melodies or chord progressions to a different key, or to see two keys simultaneously (transposing instruments!). Although there is plenty of music notation software available that can do the job, writing scores for transposing instruments and transposing songs to your or someone else’s comfortable key are skills that come in very handy and make you a very complete musician.
In this example, the inner APA shows G=do, while the outer APA shows Bb=do. If I would transpose a piece of music written in staff notation, or a chord progression written in chord symbols from G to Bb, this set-up immediately shows me what to write down.
In G major we might have the chord progression G C D G. Transposing this to Bb major would give Bb Eb F Bb. It is the same progression, in a different key. The goal of course is to gain such an understanding of tone relations and of all the keys that these steps can happen in your mind instantly!
This example shows, if we take the middle APA as concert pitch, what Bb instruments will read on their sheet music. So if you are writing parts for a tenor saxophone, for example, this set-up will show you what to write down if C is do in concert pitch.
If you were to change key to G=do concert pitch, only the MDD would rotate, leaving the inner and outer APA’s in the same relation to each other, as in this example.