Zero,

Regarding the extension of dominant harmony. As you note, there are many possibilities. The main factors that control good choices (from my experience at least) are ...

1. The melody

For example if the chord has a Bb in it and the melody has either a B natural, it's possible the chord won't sound the best.

2. The key signature

For example, if my song is in the key of F with the essential Bb, then this will impact on some chords. Let's say I wanted an A7 variation in F major. I couldn't easily choose A9 because that would potentially conflict with the Bb in the key signature; a better choice would probably be to flatten the 9th and use A7b9.

3. The chords that precede and follow the dominant harmony.

Where possible (but not always), I like to prepare a dissonance as it helps make the chord change smoother.

For example, let's say, I like the sound of A7b9 for a particular bar (my key is F major). The two dissonant notes are the 7th (G) and the b9th (Bb). For a chord that leads into the A7b9, I'd first look for something that has G and Bb in it (some possibilities relevant to F major could be C7, Bb6, Gm7, Dm6, Em7b5). If I can't find a chord that appeals, I'd then aim to find something that contains at least one of the dissonant notes.

For a chord that follows the A7b9, I keep in mind that dissonant notes usually drop by either a tone or a semitone to a note in the scale of the key signature. Thus Bb (b9th) will probably drop to A and G (7th) will probably drop to F. Suitable chords that contain both these notes are F, Dm, Dm6 (if the 9th doesn't drop but stays on Bb), Bb, etc.

The above is a bit of an overview of what goes on inside my head when I'm trying to work out how best to arrange a song.

Hope these thoughts are useful,
Noel

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