Interval reference
Augmented fourth
4 letter names · 6 semitones · C → F♯
An 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, an augmented fourth reaches F♯. Its shorthand is A4, and traditionally it counts as dissonance.
How big it is
Hear it from any note
The explorer starts on C up to F♯. Play it one note after the other and then both together — the two ways an augmented fourth turns up in music — then change the bottom note: the distance stays the same while the spelling follows the letters.
Turned upside down
Move the lower note up an octave and an 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
- The seventh of a dominant seventh chord up to its third (F up to B in G7)
- The tonic to the raised fourth of Lydian (C up to F♯ in C Lydian)
From every starting note
| From | Up an 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♯ |