I am wondering how to use the "flattened seventh" chord when used in a major key (i.e. B-flat Major chord in key C Major). I believe this would put a piece in the mixolydian mode. For example . . . I know that a diminished 7th essentially serves the same purpose as a V7 chord, but where do you go from the flattened 7th . . . how do you go to it?



In classical functional harmony, non-diatonic chords are usually diatonic to a related tonal center, so the answer is that it depends on what the local tonal center is. Normally, the ♭Ⅶ chord will be taken from the parallel minor or a related key. For example, it can be Ⅴ/♭Ⅲ, where ♭Ⅲ is the relative major of the parallel minor.

The first example that comes to mind, however, is the ♭Ⅶ in Begin the Beguine, which itself is the tonal center of the first phrase of the bridge: ⅱ/♭Ⅶ → Ⅴ7/♭Ⅶ → ♭Ⅶ. Overall, the bridge shifts the tonal center, roughly in the parallel minor, from ⅰ (the same as ⅱ/♭Ⅶ) to ♭Ⅶ to ♭Ⅵ to Ⅴ. These shifts are achieved by lowering the major third that ended the preceding phrase to the minor third:

  • ⅰ (= ⅱ/♭Ⅶ) → Ⅴ7/♭Ⅶ → ♭Ⅶ
  • ♭ⅶ (= ⅱ/♭Ⅵ) → Ⅴ7/♭Ⅵ → ♭Ⅵ7

The next chord is a diminished seventh achieved by raising the root of the preceding chord by a half step. This leads to ⅶ°7/Ⅴ and then to a period of alternation between various minor subdominant functions and the dominant, where the bridge ends. The last phrase definitively reestablishes the major key by starting with the tonic Ⅰ chord.

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    This answer is correct. I'll just add that this technique is usually referred to as "mode mixture," the chords taken from the related key are called "borrowed chords." – Peter Jul 25 '19 at 16:49

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