The diminished seventh is used in many interesting and complex ways but the basics are very simple
A diminished seventh:
- consists of a diminished triad plus a diminished seventh (hence the name)
- only appears diatonically (i.e. only using notes from one key) as chord vii7 in the minor
- resolves a semitone upwards from the root (vii7 to its tonic)
The scrunchy sound created by all those diminished intervals is unique to vii7, so it is immediately recognisable. If you hear this chord you can predict its resolution upwards by semitone (by contrast, a major chord might be followed by a wide range of different chords).
Diminished sevenths are often employed outside their diatonic context in order to highlight a particular chord. The chord vii7 is being borrowed from the relevant minor key to emphasize the chord onto which it resolves.
In Fig. 1 you can see an F# diminished seventh in its ‘home’ diatonic context as chord vii7 in G minor.
On the second stave, the same diminished seventh is borrowed to highlight chord V in C major..
Fig. 1
How to construct a diminished seventh
The best way to understand diminished sevenths is to write one. This method uses the fact that it is a borrowed vii7 to make sure we have the correct notes.
1) write down the chord to which you are resolving | In Fig. 2 this is an E major chord (V of A) |
2) write a seventh chord starting on the note a step below | In Fig. 2 we stack up thirds starting on D |
3) add accidentals that match the minor key of the chord onto which you are resolving | In Fig. 2 we are resolving onto an E chord so we put in accidentals for E MINOR (should only be be F sharps and D sharps) |
Fig. 2
Note how the diminished seventh chord resolves with its top and bottom notes resolving inwards by step.
Mozart Piano Sonata in Bb K. 333, first movement
Diminished sevenths can be used in a variety of contexts:
- to emphasize a particular chord within a key or add colour to the chord progression
- to help modulate to a new key
- as part of a chain of chromatic chords for dramatic effect
In this extract from Mozart’s Sonata K333, we are heading towards a perfect cadence in F major. In order to give the progression some direction and colour, Mozart adds a diminished seventh before the chord ii (F# diminished seventh onto G minor). The diminished seventh creates a little bit of tension that resolves onto ii before we arrive at the cadence.
