Walter Piston considers and notates viio as V07, an incomplete dominant seventh chord.
However, in modes without a leading tone, such as Dorian and Mixolydian, a raised seventh is often featured during cadences,[3] such as in the harmonic minor scale.
It functions to briefly tonicize a scale tone (usually the 5th degree)[4] as part of a secondary dominant chord.
The tritone substitution, chord progression ii–subV–I on C (Dm–Db7–C), results in an upper leading note.
According to Ernst Kurth,[9] the major and minor thirds contain "latent" tendencies towards the perfect fourth and whole tone, respectively, and thus establish tonality.
However, Carl Dahlhaus[10] contests Kurth's position, holding that this drive is in fact created through or with harmonic function, a root progression in another voice by a whole tone or fifth, or melodically (monophonically) by the context of the scale.
Forte claims that the leading tone is only one example of a more general tendency: the strongest progressions, melodic and harmonic, are by half step.
A leading-tone chord is a triad built on the seventh scale degree in major and the raised seventh-scale-degree in minor.
According to John Bunyan Herbert, (who uses the term "subtonic", which later came to usually refer to a seventh scale degree pitched a whole tone below the tonic note), The subtonic [leading-tone] chord is founded upon seven (the leading tone) of the major key, and is a diminished chord...
It is commonly used as a passing chord between a root position tonic triad and a first inversion tonic triad:[14] that is, "In addition to its basic function of passing between I and I6, VII6 has another important function: it can form a neighboring chord to I or I6.
This omission occurs, occasionally, in the chord of the dom.-seventh, and the result is a triad upon the leading tone.
"[22] In a four-part chorale texture, the third of the leading-tone triad is doubled in order to avoid adding emphasis on the tritone created by the root and the fifth.
Leading-tone seventh chords were not characteristic of Renaissance music but are typical of the Baroque and Classical period.
The example below shows fully diminished seventh chords in the key of D major in the right hand in the third movement of Mozart's Piano Sonata No.
The example below shows leading-tone seventh chords (in root position) functioning as dominants in a reduction of Mozart's Don Giovanni, K. 527, act 1, scene 13.