Diminished seventh

However, in 19 equal temperament, it is enharmonically equivalent to a augmented sixth, having a ratio of 215/19:1 (approximately 1.7284), or 947 cents.

Since the Baroque era, Western European composers have used the diminished seventh as a melodic interval to convey intense, sometimes troubled emotion.

Richard Taruskin (2010, p. 258) draws attention to the falling melodic figures in the bass (pedal) 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!

"[9] The development section features a disorienting sequence of diminished sevenths: The dramatic clout of the interval was further exploited by operatic composers during the nineteenth century.

Robert Donington (1963, p. 175) 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.