Diminished sixth

In classical music from Western culture, a diminished sixth is an interval produced by narrowing a minor sixth by a chromatic semitone. 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, despite being equivalent to an interval known for its consonance.
Its inversion is the augmented third, and its enharmonic equivalent is the perfect fifth.

"Wolf 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. Typically, this is the interval between G and E. Since this interval was considered to "howl like a wolf", and since it sounded like a badly out-of-tune fifth, this interval is called the "wolf" fifth. A justly tuned fifth is the most consonant interval after the perfect unison and the perfect octave.