Half-diminished seventh chord

In music theory, the half-diminished seventh chord play (help·info) (also known as a minor seventh flat five) is created by taking the root, minor third, diminished fifth and minor seventh (1, ♭3, ♭5 and ♭7) of any major scale; for example, C half-diminished would be (C E♭ G♭ B♭). In diatonic harmony, the half-diminished chord naturally occurs on the 7th scale tone (for example, Bm7(♭5) in C major). By the same virtue, it also occurs on the second degree of natural minor (e.g. Dm7 (♭5) in C minor).

Half-diminished seventh chords are often symbolized as a circle with a diagonal line through it, as in C^ø.

The terms and symbols for this chord break the usual system of chord nomenclature. Normally a symbol like "Bdim" indicates a diminished triad and "B7" indicates a major triad plus a minor seventh. Thus one would expect the term "Bdim7" to indicate a diminished triad plus a minor seventh. Instead, it means a diminished triad plus a diminished seventh. To make this distinction clear, the term "half-diminished" and the ^ø symbol were invented. Since the term dim7 (as in Bdim7) meant something else, the accurate but unwieldy term "minor seventh flat five" (as in Bm7(♭5)) came to be used.^[1]

Jazz musicians typically consider the half-diminished chord to be built from one of three scales: the seventh (Locrian mode) of the major scale, the sixth mode of the melodic minor scale (the latter scale is nearly identical to the Locrian mode, except that it has a natural 9 rather than a ♭9, giving it a somewhat more consonant quality), or the "half-whole" diminished scale (see octatonic scales.)

The "Tristan chord" is sometimes described as a half-diminished seventh chord; however, the term "Tristan chord" is typically reserved for a very specific harmonic function, especially determined by the order of the notes from bottom to top, and sometimes even the way the chord is spelled (e.g. is it G♭ or F♯?).

Function

The half-diminished chord has three functions in contemporary harmony: predominant function, diminished, and dominant function. The vast majority of its occurrence is on the II chord in the minor mode, wherein it takes a predominant function, leading naturally to the dominant V chord. Not including the root motion, there is only a one note difference between a half diminished chord and a V chord with a 9th. Since it is built on the diatonic II chord of the minor scale, most of the time the II-V pattern resolves to a minor tonic (such as in the progression Dm7(♭5) - G7(♭9) - Cm)., but there are instances where there is a major tonic resolution.

Diminished chord function is rarer, but it still exists. Half-diminished chords can function in the same way as fully diminished chords do, such as in the chord progression Cmaj7 - C♯dim7 - Dm7, or Em7 - E♭dim7 - Dm7, where the diminished chord serves as a chromatic passing chord preceding a chord with a diatonic root. A typical example of this is when ♯IVm7(♭5) progresses to IVm7, such as in the Cole Porter song "Night and Day", where you have the progression F♯m7(♭5) - Fm7 - Em7 - E♭dim7 - Dm7 - G7 - Cmaj7. If it was analyzed in its predominant function, it wouldn't be a sufficient explanation to how it functions preceding the Fm7 chord.

In dominant function, the VII half diminished chord, like its fully diminished counterpart, can take the place of the dominant V chord at a point of cadential motion. This generally occurs in a major key since the minor key has a flattened 6th scale degree which would make it fully diminished. Indeed, the VII half diminished chord in a major key is identical to a dominant ninth chord (a dominant seventh with an added ninth) but with its root omitted.

Example

Bmin7♭5 chord - midi audio

References

1. ^ Mathieu, W.A. Harmonic Experience: Tonal Harmony from Its Natural Origins to Its Modern Expression (1997), pp. 371-372, Inner Traditions International, ISBN 0-89281-560-4

External links

• Improvising Over Half Diminished Chords