Q: What would you consider to be the "dominant" and the "subdominant" chords in the Phrygian mode? Is there a "leading tone" chord i.e. in Major, we have diminished chords; what is there in Phrygian?
A: Regarding the modes, I don't imagine it useful think in diatonic systems so to speak. This works great for the major and minor scales but not so well for the modes. Regarding the harmonized major scale, a I-IV-V or Tonic-Subdominant-Dominant progression (C-F-G or Cmaj7-Fmaj7-G7), harmonically has a lot of strength, it ends with a perfect cadence, meaning a major or dominant chord resolving to a major tonic chord. But if we were to use the same system (using C phrygian for example), making the iii chord (of Eb major), the tonic or i chord, we would get a Cmin-Fmin-Gdim progression (i-iv-vo), which is rather lame. Yikes! A diminished vo chord!
You sort of have to change your way of thinking when dealing with modes. Modes don't work well harmonized for long progressions. Usually one chord for a measure or a few measures leading to something completely unrelated is the norm. It is more important to think more about the quality of the chord and pay close attention to what the chord is with its "modal" extensions added on. In other words, a measure of a Csus(b9) chord is a way more effective use of modal harmony than a long chord progression of chords diatonic to C phrygian (Eb major). Sometimes shorter progressions work fine. As an example, a Cmin-Db or Cmin7-Dbmaj7 chord progression works fine for phrygian.
I suppose that a Cmin-Dbmaj-Bbmin progression would work. But in all reality the more chords you were to add in, the weaker the phrygian tonality would become. If you really wanted a phrygian type sound, it would seem best to use a sus(b9) chord. Let's say you have a C progression you are working in and you wanted to work in a phrygian sound, it would seem better for you to replace the I chord with a Csus(b9) chord to create a real phrygian texture. You may even extend it for a bar of two or create a vamp before falling back to your C major progression.
Regardless, you should experiment. If you wanted to harmonize the C phrygian scale, this is what you would get:
Cmin-Db-Eb-Fmin-Gdim-Ab-Bbmin
If you were to consider these chords as borrowed from phrygian and imported them to C major you would get this: i-bII-bIII-iv-Vo-bvi-bvii
You do sort of see this thing from time to time. You often get the bII and bIII in major keys and of course the minor iv chord (although the latter two usually considered borrowed from C minor).
Experiment and see what your ear likes.
-Chris
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