Because of this it can also be viewed as four notes all stacked in intervals of a minor third. The diminished seventh contains two diminished fifths, which often resolve inwards.
In most sheet music books, Cdim or C° denotes a diminished seventh chord with root C; but it may also happen, mostly in modern jazz books and some music theory literature, that Cdim or C° or Cm(♭5) denotes a diminished triad, while Cdim7 or C°7 or Cm6(♭5) denotes a diminished seventh chord.
Diminished seventh chord resolution: both diminished fifths tend to resolve inward, doubling the third of the tonic chord Play (help·).
The most common form of the diminished seventh chord is that rooted on the leading tone; for example, in the key of C, the chord (B, D, F, A♭). So its other constituents are the second, fourth, and flatted sixth (flat submediant) scale degrees. These notes occur naturally in the harmonic minor scale. But this chord also appears in major keys, especially after the time of Bach, where it is "borrowed" from the parallel minor. Fétis tuned the chord 10:12:14:17.
Seventh chords may also be rooted on other scale degrees, either as secondary function chords temporarily borrowed from other keys, or as appoggiatura chords: a chord rooted on the raised second scale degree (D♯-F♯-A-C in the key of C) acts as an appoggiatura to the tonic (C major) chord, and one rooted on the raised sixth scale degree (A♯-C♯-E-G in C major) acts as an appoggiatura to the dominant (G major) chord. Because these chords have no leading tone in relation to the chords to which they resolve, they can not properly have "dominant" function. They are therefore referred to commonly as "non-dominant" diminished seventh chords or "common tone" diminished seventh chords. (See "common tone diminished seventh chord" below)
In jazz, the diminished seventh chord is often based on the lowered third scale degree (the flat mediant) and acts as a passing chord between the mediant triad (or first-inversion tonic triad) and the supertonic triad: in C major, this would be the chord progression E minor - E♭ diminished - D minor. The chord, "plays no role in...jazz." The passing chord is used widely in Brazilian music like Choro, Samba, and Bossa Nova
The diminished seventh chord normally possesses a "dominant" function, and this is most straightforwardly shown when the root of a dominant seventh chord is omitted. The remaining third, fifth and seventh of that chord form a diminished triad (whose new root is the third of the former chord), to which a diminished seventh can be added. Thus in C (major or minor), a dominant seventh chord consisting of G, B, D, F can be replaced by a diminished seventh chord B, D, F, A♭. (In jazz harmony, a combination of the original chord with its substitute (with G in the bass and Ab simultaneously in an upper voice) yields the very common "7♭9" chord, which intensifies the dominant function of either a diminished seventh or dominant seventh chord.) Other transformations of this kind facilitate a variety of substitutions and modulations: any of the four notes in a diminished seventh chord are raised by a semi-tone, that raised note is then the flat-seventh of a half-diminished seventh chord. Similarly, if any of the four notes in the diminished seventh chord are lowered by a semi-tone, that lowered note is then the root of a dominant seventh chord.
The diminished seventh chord comprises frequencies that are equally spaced when considered on a logarithmic axis, and thus divides the octave into four logarithmically equal portions, each being a minor third.
Two diminished seventh chords in the octatonic scale (one red, one blue) may be rearranged into the alpha chord Play (help·).
The diminished scale may be conceived of as two interlocking diminished seventh chords, which may be rearranged into the alpha chord.
Sharpened subdominant with diminished seventh
Sharpened subdominant with diminished seventh chord in C. Play (help·)
The sharpened subdominant with added diminished seventhchord is another common use of the chord. It can be simply represented with the Roman notation ♯ivo7, but in classical music is more correctly represented as viio7/V, being a very common way for a composer to approach the dominant of any key. In the key of C, this is F♯dim7, which may be used for a strikingly fearful effect, as its root is a tritone (augmented fourth) from the tonic. It is also a common chord in jazz and ragtime music. A common traditional jazz or Dixieland progression in E♭ may go:
Sharpened subdominant with diminished seventh chord progression in C Play (help·).
♯IIo7 as dominant substitute with III-7 substituted for the tonic (I) chord (D♯-E) Play (help·).
Common-tone diminished seventh chord
Common-tone diminished seventh chord Play (help·).
A diminished seventh chord may alternatively resolve to a major or major-minor seventh chord whose root is one of the notes of the diminished seventh chord, the most common being the raised supertonic seventh, which resolves to the tonic in major keys and the raised submediant, which resolves to dominant triad or seventh in major keys, with the altered tones resolving upward by half step.
Diminished seventh chord to dominant cadence (bo-e7-A7-D) Play (help·).
The diminished chord may also resolve through lowering two of the bottom three voices producing a supertonic seventh chord that may lead to a conventional dominant cadence.
Diminished seventh root
In Rameau's supposition the root of the dominant chord on B, left, is substituted producing a diminished seventh chord on C, right.
Music theorists have struggled over the centuries to explain the meaning and function of diminished seventh chords. Currently, two approaches are generally used. The less complex method treats the leading tone as the root of the chord, and the other chord members as the third, fifth, and seventh of the chord, the same way other seventh chords are analyzed.
Diminished seventh chord incomplete ninth in C Minor Play (help·).
The other method is to analyze the chord as an "incomplete dominant ninth", that is a ninth chord with its root on the dominant, whose root is missing or implied. A vii°7 chord in the minor key (for example, in C minor, B♮, D, F, A♭) occurs naturally in the harmonic minor scale and is equivalent to the dominant 7(♭9) chord (G, B, D, F, A♭) without its root. This was already proposed by Arnold Schoenberg, and Walter Piston has long been the champion of this analysis. Jazz guitarist Sal Salvador, and other jazz theorists, also advocated this view, rewriting chord charts to reflect this and supplying the "missing" root as part of their bass lines.
The dominant ninth theory has been questioned by Heinrich Schenker. He explained that although there is a kinship between all univalent chords rising out of the fifth degree, the dominant ninth chord is not a real chord formation.
Rameau explained the diminished seventh chord as a dominant seventh chord whose supposedfundamental bass is borrowed from the sixth degree in minor, raised a semitone producing a stack of minor thirds. Thus in C the dominant seventh is G7 (G-B-D-F) and the sixth degree borrowed from minor produces A♭-B-D-F. He observed in his Treatise on Harmony that three minor thirds and an augmented second make up a chord where the augmented second is such that "the ear is not offended" by it. He may have been talking of the augmented second in quarter-comma meantone, a tuning he favored, which is close to the just septimal minor third of 7/6.
The fundamental tone or root of any diminished seventh chord, being composed of three stacked minor thirds, is ambiguous. For example, Cdim7 in root position: C + E♭ + G♭ + B (each has one and half interval), is just as easily viewed as an E♭dim7 in its third inversion:
D (enharmonic equivalent of C) + E♭ + G♭ + B.
It can also be viewed as a G♭dim7 in its second inversion:
D + F (enharmonic equivalent of E♭) + G♭ + B.
Delineating this chord in its last possibility, that of Bdim7 in its first inversion, is very clumsy and not very useful as it requires the use a triple-flatted note, something that is hardly ever used in a musical score:
D + F + A (enharmonic equivalent of G♭) + B.
However, by enharmonically respelling the B to A, this can also be viewed as a first inversion Adim7 chord:
C + E♭ + G♭ + A (enharmonic equivalent of B).
Other possibilities present themselves by respelling the various roots; for instance:
C + E♭ + F♯ (enharmonic equivalent of G♭) + A (enharmonic equivalent of B) (second inversion F♯dim7).
C + D♯ (enharmonic equivalent of E♭) + F♯ (enharmonic equivalent of G♭) + A (enharmonic equivalent of B) (third inversion D♯dim7).
B♯ (enharmonic equivalent of C) + D♯ (enharmonic equivalent of E♭) + F♯ (enharmonic equivalent of G♭) + A (enharmonic equivalent of B) (root position B♯dim7).
All of the chord's inversions have the same sound harmonically. Because of the chord's symmetrical nature (superimposing more minor thirds on top of the dim 7 produces no new notes), there are only three different diminished seventh chords possible.
The diminished seventh chord can appear in first, second, or (least common) third inversion. Each inversion is enharmonic with another diminished seventh chord, and 19th-century composers in particular often make use of this enharmonic to use these chords for modulations. Percy Goetschius calls it the "enharmonic chord."
Diminished seventh chord on C, written four different ways enharmonically (all sounding the same).
Using Piston's incomplete-ninth analysis, a single diminished seventh chord, without enharmonic change, is capable of the following analyses: V, V of ii, V of III (in min.), V of iii (in maj.), V of iv, V of V, V of VI (in min.), V of vi (in maj.), V of VII (in min.). Since the chord may be enharmonically written in four different ways without changing the sound, we may multiply the above by four, making a total of forty-eight possible interpretations. More conservatively, each assumed root may be used as a dominant, tonic, or supertonic, giving twelve possibilities.