Tritone

The tritone is an interval of 6 semitones — exactly half an octave — also called the augmented 4th (♯4), diminished 5th (♭5), or "the devil's interval". It is the most dissonant of the simple intervals: restless and unstable, and the only interval that inverts to itself. It is the signature interval of lydian (♯4) and locrian (♭5), and the defining tension of the dominant 7th chord (between its 3rd and ♭7), which drives the V–I resolution underpinning much of Western harmony.