18c Lesson - Common-tone diminished chords

Class Discussion

Common tone diminished chords

If you have a C chord, voiced C,G,E,G; and it moves to a series of pitches, voiced C,F#,D#,A. It moves back to the same voicing of C.

• you could call it a D#o7, but it doesn’t function as a viio7/iii because it doesn’t move to the iii.
• you could call it an Ao7 if you respell two of the notes, but that’s a #vio7, which isn’t borrowed from any mode.

How does that use of pitches relate to the I chord? It creates a diminished chord where all the pitches are tendency tones to the I chord, while carrying the tonic through it. In C, a common tone diminished chord is spelled C, D#, F#, A. That’s a C (tonic) with a diminished chord on top (D#o).

• the common-tone is usually the root for voice leading reasons, but it doesn’t have to be.

Common-tone diminished chords have tonic function and serve to elongate the tonic function. It decorates a major I chord. These don’t exist in minor. Ideally, the I chord exists on either side of the common-tone diminished chord (I - cto - I), but that doesn’t always happen.

• take the chord progression from the third example: V6/V - cto - V43/V.
  • the cto is used to decorate the V/V with chromatic motion
    • you don’t have to identify the chromatic tones as non-chord tones because you’re classifying it as a common-tone diminished chord.