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diminished fifth
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diminished fifth (plural diminished fifths)

1. (music) A type of tritone in musical composition comprising six semitones. It spans five diatonic scale degrees like a perfect fifth but narrower by a semitone. The number of diatonic degrees it spans is what distinguishes it from an augmented fourth.

Synonym: flatted fifth
C - G is a perfect fifth, C - G♭ is a diminished fifth, and C - G# is an augmented fifth.