19b Discussion - Common-tone diminished chords

Class Discussion

Common Tones Diminished 7th Chords

A common tone diminished 7th chord is a fully diminished seventh chord that shares a tone between the chord before it, and the chord after it.

You can think of them as a cluster of non-chord tones: one tone will remain the same (this is the common tone) and act like a pedal tone, and the other three notes will act like neighbor tones or passing tones.

These chords are always labeled as cto7, no matter what it resolves to or how it’s spelled.

Put the leadsheet as you normally would. Take care to notice how it is spelled.

Class Discussion 2022

Common tone diminished chords

• These are similar to +6 chords in that they don’t quite work right. These also exist just as decoration/voice leading into the next chord. It’s made up entirely of tendency tones to make the resolution really strong
• Confusing to ID because they look like another chord
  • Whether a chord is a ct07 depends on what comes after it. If we’re in C and we have a D#07/C, it can be either a ct07 or a vii07/iii. If the next chord is iii, it’s the latter. If it’s I, the chord is a ct07.
  • The other common Roman numeral that comes after a ct07 is V
  • Do not label inversions of ct07s. Like +6 chords their function doesn’t work with our system of inversion labeling

Constructing a ct07

• A ct07 going to I will look like a #ii07, while a ct07 going to V will look like a #vi07. These two points of reference are helpful for knowing what notes you need, but be aware that writing these in “root position” will be incorrect
  • Again: these chords exist for cool voice leading! If you just write it in root position, all of that cool voice leading goes away. One way to ensure the voice leading is correct is to write the following chord next to it in root position, then construct your ct07 from there by determining what all the notes should resolve to

These are also used in enharmonic modulations and you just label them as ct07 like usual