In classical music from Western culture, a diminished seventh is an interval produced by narrowing a minor seventh by a chromatic semitone. For instance, the interval from A to G is a minor seventh, ten semitones wide, and both the intervals from A to G, and from A to G are diminished sevenths, spanning nine semitones. Being diminished, it is considered a dissonant interval. The diminished seventh is enharmonically equivalent to a major sixth. Its inversion is the augmented second. The diminished seventh is used quite readily in the minor key, where it is present in the harmonic minor scale between the seventh scale step and the sixth scale step in the octave above. In an equal tempered tuning, a diminished seventh is equal to nine semitones, a ratio of 29/12:1, or 900 cents. There is no standard just tuning of this interval, but one possibility, assuming the flat submediant is a perfect major third below the octave, and the leading tone to be 15:16, would lead to an interval of 128:75, about 925 cents; another interval is 216:125, which is three minor thirds. The 128:75 just diminished seventh arises in the C harmonic minor scale between B and A by combining B-D, D|F, F-A.
Since the Baroque era, Western European composers have used the diminished seventh as a melodic interval to convey intense, sometimes troubled emotion. Richard Taruskin draws attention to the falling melodic figures in the bass part of J. S. Bach's organ chorale prelude from the Orgelbüchlein, "Durch Adam's Fall":] “What is a powerful surprise, and further evidence of Bach’s unique imaginative boldness, is the specific form the obbligato pedal part takes in this chorale setting: almost nothing but dissonant drops of a seventh – Adam’s fall made audible! And not just the fall, but also the attendant pain and suffering are depicted, since so many of those sevenths are diminished.” The expressive potential of the interval was not lost on Mozart, especially when, as Dennis Matthews puts it, “Mozart reserved a minor home-key for his most disquieting utterances.” The opening theme of the Piano Concerto in C minor K491, features rising diminished sevenths:] More startling is the finale of Mozart's G minor Symphony No. 40, K550, which critic Charles Rosen calls "a work of passion, violence and grief." The development section features a disorienting sequence of diminished sevenths:File:Mozart K550 finale bars 125-30.png|thumb|center|500px|Mozart, Symphony No. 40, finale, bars 125–130. The dramatic clout of the interval was further exploited by operatic composers during the nineteenth century. Robert Donington heard the dark, atmospheric Prelude to Wagner’s opera Siegfried as “a kind of elemental brooding…Its material is an uneasy sequence of thirds low in the bass, and separated by a diminished seventh.”:]
