Interval reference
Augmented fourth
4 letter names · 6 semitones · C → F♯
A augmented fourth spans 4 letter names and 6 semitones. Both halves of that description matter: the number comes from the letters, the quality from the exact size.
Starting on C, a augmented fourth reaches F♯. Its shorthand is A4, and traditionally it counts as dissonance.
How big it is
Turned upside down
Move the lower note up an octave and a augmented fourth becomes a diminished fifth. The two numbers add up to nine, and the quality flips — major with minor, augmented with diminished, while perfect stays perfect.
Where you meet it
- Between the third and seventh of a dominant seventh chord
- The Lydian raised fourth
From every starting note
| From | Up a augmented fourth |
|---|---|
| C | F♯ |
| C♯ | F𝄪 |
| D | G♯ |
| D♯ | G𝄪 |
| E | A♯ |
| F | B |
| F♯ | B♯ |
| G | C♯ |
| G♯ | C𝄪 |
| A | D♯ |
| A♯ | D𝄪 |
| B | E♯ |