Augmented octave

In Western tonal music theory, an augmented octave is the sum of a perfect octave and an augmented unison or chromatic semitone. It is the interval between two notes, with the same note letter on staff positions an octave apart, whose alterations cause them, in ordinary equal temperament, to be thirteen semitones apart. In other words, it is a perfect octave which has been widened by a half-step, such as B♭ and B♮ or C and C♯; it is a compound augmented unison.^{[citation needed]} It is the enharmonic equivalent of a minor ninth.^[1]

1. ^ William Drabkin, "Octave (i)", in Grove Music Online. Oxford Music Online. Accessed February 2012. (subscription needed)

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