18b Discussion - Common-tone diminished chords

Common-tone Commotion

Like augmented sixth chords, these are a collection of tendency tones.

Where it gets its name:

• A common tone diminished 7th chord is a fully diminished seventh chord that shares one note with the chord before it and the chord after it. This shared note acts like a pedal tone.

Uses:

1. Going from a I chord to a ct^o7, then back to I.
2. Going from V or V⁷ to a ct^o7, then back to V or V⁷.

How they’re made:

• Write in the root of the chord that comes before and after it. This is the common tone that will remain the same throughout.
• Add in the other three notes that make it a fully diminished 7th chord.
• These other notes must be spelled in a way that the accidentals resolve properly to their appropriate notes.
• Ex: a ct^o7 chord that is between two A major chords will be spelled A, B^#, D^#, F^#. The A resolves to A, the B^# resolves to C^#, the D^# resolves to E, and the F^# reolves up to A. (If it were resolving to an A⁷ chord, the F^# would resolve to G)
• So, don’t be fooled! The common tone is never the actual root of the diminished chord.

Shortcuts:

• ct^o7 of I is a #ii^o4/2
• ct^o7 of V is a #vi^o4/2

Labeling:

• The leadsheet will be normal, identifying exactly what diminished chord it is.
• Instead of a roman numeral, it will be labeled as ct^o7 below the staff. There are no inversion of this!