Augmented fourth
The augmented fourth spans six semitones and is a perfect fourth widened by a semitone. Together with the diminished fifth it forms the tritone. On tempered instruments both sound the same, but the spelling says something about origin and continuation.
- Semitones
- 6
- Example
- F–B
- Sounds the same as
- diminished fifth (B–F)
- Inversion
- diminished fifth
- Major scale
- between the fourth and seventh degrees
Six semitones
The perfect fourth has five semitones, for example C–F. If you widen it by a semitone the augmented fourth arises: C–F♯ or F–B. That is three whole tones in a row, hence the name tritone. The intervals page of the site explains why it was called “diabolus in musica”: Intervals.
Augmented fourth against diminished fifth
On the left F–B: four steps, so a fourth, augmented because it spans six semitones. On the right F–C♭: five steps, a fifth, diminished. Both sound the same on the piano (F and B on the white keys against F and C♭, which is also B). Which spelling applies is decided by the harmonic context.
Resolution of the augmented fourth
The augmented fourth F–B strives outward: F falls to E, B rises to C, and the minor sixth E–C arises. The diminished fifth B–F, by contrast, contracts: B rises to C, F falls to E, and the major third C–E arises. The same notes, two directions.
In the glossary: Tritone · Interval inversion
Fourth or fifth?
The difference from the diminished fifth can be read off in the notation from the number of steps: F–B spans four steps (F, G, A, B), F–C♭ five (F, G, A, B♭, C♭). Which spelling is chosen depends on the harmonic context and the resolution.
| Augmented fourth | Diminished fifth | |
|---|---|---|
| Example | F–B | B–F |
| Steps | four | five |
| Semitones | 6 | 6 |
| Tendency | notes strive apart | notes contract |
Where to find it
In the major scale the augmented fourth arises between the fourth and the seventh degrees, in C major between F and B. Both notes belong to the dominant seventh chord G7, in which B (third) and F (seventh) form a tritone. That is the core of the tension that resolves in the tonic.
In the Lydian mode the augmented fourth lies between the tonic and the fourth degree, in F Lydian between F and B. This raised fourth degree shapes the open, wide sound of the mode. In altered chords and secondary dominants too the interval is familiar.
Historically the tritone was to be avoided in the melodic line. Musica ficta served, among other things, to avoid it by raising or lowering a note.
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Related terms
TritoneInterval inversionLydianDominantSeventh chordAugmented second
Editor: Maik Krohm · reviewed on 3 October 2026
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