Some chord transitions

Let's say you're playing in the key of C major.

Start on C.maj, augment the root chord tone, now you've got C#.dim, which isn't diatonic but it moves nicely to D.min chord. It's like a chromatic bassline connection between diatonic chords. And the C#dim notes are all in A.7, which is the secondary dominant of D.min, so you can play (C.maj -> A.7 -> D.min) and get an almost identical motion.

If you want to do a (I II V I) progression with even more secondary dominants, then you've got 

    [C.maj -> (A.7) -> D.m7 -> (D.7) -> G.7 -> C.maj]

And all of those .7 and .m7 chords can become .9 and .m9 chords if you've got the fingers for it. I've been putting 9th degrees on chords lately and enjoying the sound of it a lot, but the chord functions mostly only depend on degrees ^1, ^3, ^5, ^7, so that's how I'll write it in progressions.

If instead of resolving (I II V I) to I, you go to VI, which also has a dominant function, then you can sneak in one more secondary dominant:

    [C.maj -> (A.7) -> D.m -> (D.7) -> G.7 -> (E.7) -> A.m]

And if you remove root notes from the secondary dominants, then you've got nice diminished passing chords still.

    [C.maj -> (C#.dim) -> D.m -> (F#.dim) -> G.7 -> (G#.dim) -> A.m]

I've started forming a mental inventory of when you can functionally play one triad over another (or one 7th chord over another). For example, if you put a G.min over C.maj, you get a C.9 chord. What can you do with it? Well, C.9 is C.7 with an extension, and C.7 is a secondary dominant to F.maj, which is diatonic. So follow C.9 with F.maj or F.maj7 or F.maj9 and that will sound functional.

When I'm playing jazzy chords, neither of C.maj7 and C.6 sound as resolved as C.maj, but C.maj doesn't sound jazzy, so I'm never sure how to end my phrases. Often I switch between C.maj, C.maj7, C.6, making a little melody on top of the chord. Another option is to play C.maj9. How does adding an extension make C.maj7 sound more resolved? I think it's because .maj9 is used a lot as a resolution chord in bossa nova. It's less that the chord is resolved in terms of its spectral/harmonic content and more that there's a cultural idiom that this is a nice stopping place. Alternatively C.maj9 is like G.maj over C.maj, and maybe each of the two diatonic major chords each give you some harmonic resolution.

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I knew most of these things theoretically in the past, but lately I've played playing enough piano that they're becoming engrained, and I maybe writing about them again now that they're more fully a part of me will improve the presentation.