Diminished octave

In music from Western culture, a diminished octave (Play^ⓘ) is an interval produced by narrowing a perfect octave by a chromatic semitone.^[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.^[3]

diminished octave

Inverse	Augmented unison
Name
Other names	Diminished eighth
Abbreviation	d8[1]
Size
Semitones	11
Interval class	1
Just interval	48:25, 49:26 (13-limit), 256:135,[2] 4096:2187
Cents
12-Tone equal temperament	1100[2]
Just intonation	1129, 1108,[2] 1086

Diminished octave on C Play^ⓘ.

Diminished octave on C-sharp Play^ⓘ.

The diminished octave is enharmonically equivalent to the major seventh.