The augmented fourth (aug4), famously known as the tritone, is the most dissonant and unstable interval in Western music. Spanning 6 semitones, it perfectly bisects the octave, creating a restless, "symmetrical" tension that has earned it the nickname "the Devil in music" (diabolus in musica).
Construction and spelling
An augmented fourth occurs between a root and its fourth letter name when the interval is raised by a semitone, such as C to F♯ or B♭ to E. It is enharmonically identical to the diminished fifth (♭5), but its spelling dictates its function: an aug4 usually resolves outward to a sixth, whereas a ♭5 resolves inward to a third.
Harmonic and melodic usage
Harmonically, the aug4 is the "engine" of the dominant seventh chord. It exists between the 3rd and the 7th of the chord, creating the intense instability that forces the ear to crave a resolution to the tonic. Melodically, the tritone is famously difficult to sing and was historically avoided, but it is now a staple of jazz, heavy metal, and contemporary film scores for its angular, mysterious, and aggressive character.
Examples
- The opening interval of the Simpsons theme (C to F♯)
- The main riff of Black Sabbath’s "Black Sabbath" (G to C♯)
- The "Maria" motif from West Side Story (the lift from the first to the second syllable)
- The defining tension in a G7 chord (B to F) that pulls toward C Major
In practice
To master the augmented fourth, practice resolving it to a major sixth (e.g., F–B resolving to E–C). Because of its symmetry, any tritone can resolve in two opposite directions, which is the secret behind tritone substitution in jazz.
When improvising, use the aug4 to add a "modern" or "Lydian" brightness over major chords, or use it to create dark, grinding tension in rock and metal. Its high-energy dissonance makes it the perfect tool for signaling a change in harmonic direction or mood.
