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I recently found out the auxiliary diminished chord, and that I can play it over a dominant chord, but I'm not really familiar with it, I'm still trying to understand it, so there might be some mistakes below:

When a guitarist and a bassist(me) play a simple II-V-I like Dm7 | G7 | Cmaj7 | Cmaj7 and over the G7 chord, the guitarist (which is soloing) is playing an auxiliary diminished (G#) scale which only has a different note from G7 (G# instead of G natural), can I (the bassist) play that note (b9 -- G#/Ab) on my walking bass lines? Or Ι should try avoiding the b9?

Update: I made myself a bit more clearer.

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up vote 4 down vote accepted

First of all, you should notice that if you superimpose G7 and G#dim, the resulting chord is G7(b9) (enharmonically spelling G# as Ab). So, you do not need to think of it as separate chords. That said, you should be encouraged to explore every note of the chord. As an overall suggestion, on popular music it is often ineffective to use alteration notes (such as the b9) over strong beats (as someone said above, times 1 and 3 if you have a 4 time meter). Aside from that, you would be safe starting by using G7 chord notes on strong beats, and gradually proceed into using chromatic leading tones when preparing those beats (A# -> B, C# -> D and so on). Theoretically, you can use any tone which is close enough to chromatically lead into strong beats (those will generally sound better with standard chord notes, but why not to try something different?).

Practice and rehearsal will tell you which combinations better suit the overall sonority of the piece you are to play. When you face such chord superimpositions, try to figure out which is the resulting sonority, highlight the main chord notes and approximate them chromatically. You start safe and can end up with awesome!

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When one plays a walking bass, one uses all sorts of notes that are not necessarily included in the underlying chord. It is not just going up/down an arpeggio. In 4 time, generally 1st and 3rd beats will be played on notes from the chord, but not always.

The dominant chord is there to take the music back to tonic (home), so the G is the 'right' bass note to play. However, the underlying chord (with an Ab) is rather like a G7b9 here, which would include the G anyway. Sometimes the G works better as the bass note, sometimes Ab. It rather depends on the guitarist's voicing of the chord, and the melody on top.

With walking bass, for a reason that escapes me right now, the last note in a previous bar can be the leading note of the next chord root, or a note one semitone above.As in the ii bar's last note could be that Ab anyway - even though Ab doesn't feature in Dm or G7. Theoretically it shouldn't work, but it does sound effective.

As with most music playing, if what you do sounds good, it probably is; if it sounds duff, it probably is.

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The 'leading note' you reference (not to be confused with the leading tone) would be called an approach note and can also be done from below (like a leading tone). It works because the tension it creates desires resolution to the following root. This is a great device for changing directions of walking or for making significant register changes, which is generally avoided while walking. Most commonly when making this register change, the resolution will be in the opposite direction; if you leap downward, it would usually be a half-step below your note of resolution and vise versa. – Basstickler Dec 26 '13 at 14:27
Thanks. 'Leading note'- English - a note one semitone lower (or higher) than the target - usually tonic.That's why minor stuff raises the normally flattened 7th note, to create the push better. I still don't know how the higher leading note works, though. The U.S. term leading tone is a great misnomer - it's really a semitone ! – Tim Dec 26 '13 at 15:00
Yes, in the US the leading tone is the major 7. I relate the higher approach note (leading note in English terms) to the way the 4th degree of a scale (major) wants to resolve down by 1/2 step to the 3rd, often times featured as the 7 of a V7 chord. Looking at the major scale, there are 2 half-steps, which are the key to V-I resolution; 7->1, 4->3. As you mentioned, the 7 is raised in minor to push better but it's also notable that resolving to minor is less strong because of the whole step resolution down to minor 3. I believe this is part of the reason that the Picardy Third came to be. – Basstickler Dec 26 '13 at 16:10
Interesting. But the Picardy third only occurs as the final cadence. If the pull was that strong, maybe it would come elsewhere in a piece. But then, it would have modulated to the parallel major. – Tim Dec 26 '13 at 16:18
Yeah, I was running out of room in my comment (I tend to be thorough). I think the Picardy third came into existence because the resolution to minor felt somewhat weak and made the piece or movement feel less finished at the end. By making the final chord major you get a stronger resolution, therefore more finality. The other reason I imagine it came into play is the fact that a major chord is more consonant than a minor chord, which also gives a stronger sense of finality. – Basstickler Dec 26 '13 at 16:38

Yes, you can use Ab for 3 first beats, but on 4th (last) it's better to get back to G (or equivalent substitution) for clearly point the dominant to C.

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