A diminished seventh chord may sound dissonant at first glance, but when utilized correctly it can add tension and excitement to any song.
Diminished seventh chords are unique because they’re symmetrical; every note except the leading tone can be changed to any other note to achieve four distinct notes that make up this chord.
Root Note
The diminished seventh chord stands out among other chords because any note can act as its root note, making it ideal for modulation purposes. Furthermore, its four different interpretations and functions make it very flexible as an adaptable pivot chord.
The diminished seventh is often used to tonify other notes than its tonic, either briefly without actually leaving its home key, or as part of modulation into another key – this type of function is known as secondary leading tone chord.
Diminished seventh chords can also serve as V chords with their roots removed, creating what’s known as a rootless diminished seventh progression that leads to either vio7 chords or iii chords; sometimes even moving up an extra major second for an unnatural sound and movement in music pieces.
Scale Intervals
Diminished seventh chords have translational symmetry, meaning you can move their highest note up or down an octave from its original position without changing their shape or meaning. This makes diminished seventh chords ideal tools for transposition and substitution purposes.
As with triads, each interval quality in diminished seventh chords is represented by its own pair of note names; thus allowing diminished seventh chords to have three root notes (representing their lowest notes) which are usually known by their names in order to show how each note above the root differs in quality compared to one another as illustrated in the table below.
Example 1 – An A diminished 7th chord is composed of its root A and two minor thirds Ab and Eb stacked one upon another, while Example 2 (an Eb diminished 7th) uses a triad with its major sixth replaced with an augmented second Gb.
Triads
Subtracting its root note from a seventh chord built on sol (5) results in the formation of a diminished triad, made up of A, C & Eb notes. When this combination is added onto a minor diminished seventh chord it forms half-diminished seventh chords.
Similar chords built upon re and ti are composed of major triads with minor sevenths, while those comprised by diminished triads feature diminished sevenths instead.
These chords possess a fluid quality that makes them suitable for modulating between keys. Lowering any one of the triad notes by one semitone will produce a dominant seventh chord belonging to that new key.
Diminished seventh chords are popular transitional chords due to their dissonant characteristics. Furthermore, they serve as common tone modulators (CTM) for creating key changes by modulating tone with its inherent dissonance.
Thirds
As our final chord, we will explore a fully diminished seventh. To construct this chord, add a diminished triad to the root of a dominant seventh chord and change its major to diminished status by lowering both second and fourth notes by one half step each (to C and E respectively) until we obtain Adim7 on the staff diagram.
As opposed to its ascending diminished counterpart, which often serves as an effective solution chord in certain circumstances, descending diminished is less suitable as a resolution chord due to the two tritones being highlighted by bass; they must therefore be handled carefully when used.
One unique quality of the diminished seventh chord is its versatility in terms of root note choice; since all intervals are one and one half tones apart. Therefore, we could spell it Cdim7 (C, Eb, Gb, A), Gdim7 (G, B, C, E) or even Ddim7 (D, C, E & G).