In classical music from Western culture, a diminished sixth (Play^ⓘ) is an interval produced by narrowing a minor sixth by a chromatic semitone.^[1][3] For example, the interval from A to F is a minor sixth, eight semitones wide, and both the intervals from A♯ to F, and from A to F♭ are diminished sixths, spanning seven semitones. Being diminished, it is considered a dissonant interval,^[4] despite being equivalent to an interval known for its consonance.

Its inversion is the augmented third, and its enharmonic equivalent is the perfect fifth.

A severely dissonant diminished sixth is observed when the instrument is tuned using a Pythagorean or a meantone temperament tuning system. Typically, this is the interval between G♯ and E♭. Since it seems to "howl like a wolf" (because of the beating), and since it is meant to be the enharmonic equivalent to a fifth, this interval is called the "wolf" fifth. Notice that a justly tuned fifth is the most consonant interval after the perfect unison and the perfect octave.