Diminished sixth

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.

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

A severely dissonant diminished sixth is observed when a fixed-pitch instrument limited to twelve notes per octave is tuned using Pythagorean tuning or a meantone temperament with a fifth flatter than 700 cents.

Since this interval was considered to "howl like a wolf" (because of the beating), and since it sounded like a badly out-of-tune fifth, this interval is called the "wolf" fifth.