Diminished octave
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In music from Western culture, a diminished octave (ooui-php-3Play) is an interval produced by narrowing a perfect octave by a chromatic semitone.cite-ref-b-s-1-1[1] As such, the two notes are denoted by the same letter but have different accidentals. For instance, the interval from C4 to C5 is a perfect octave, twelve semitones wide, and both the intervals from C♯4 to C5 and from C4 to C♭5 are diminished octaves, spanning eleven semitones. Being diminished, it is considered a dissonant interval.cite-ref-3[3]
The diminished octave is enharmonically equivalent to the major seventh.
References
cite-note-b-s-11. Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p.54. ISBN 978-0-07-294262-0. Specific example of an d8 not given but general example of perfect intervals described.
cite-note-duffin-21. citerefduffin2008Duffin, Ross W. (2008). How equal temperament ruined harmony : (and why you should care) (First published as a Norton paperback. ed.). New York: W. W. Norton. p. 163. ISBN 978-0-393-33420-3. Retrieved 28 June 2017.
cite-note-33. ↑ Benward & Saker (2003), p.92.