Introduction
A perfect fourth is five semitones. It is one of the three perfect intervals (along with the unison and fifth) and was treated as a stable consonance in medieval music. In modern theory it is a complicated case — consonant on its own, but considered a dissonance when it appears above the bass in counterpoint.
How to Find It on the Keyboard
Find any perfect fourth in two simple steps. The number tells you the letter. The semitones tell you the accidental.
- Start on any root note. Count 4 letter names (including the root) up the musical alphabet — that gives you the top letter.
- Now count exactly 5 semitones from the root. If the natural top letter is too high or too low, sharpen or flatten it to land on the right pitch.
- Use the explorer above to check yourself in all 12 keys. The two highlighted notes are the P4 from that root.
Quick check: from C, the P4 lands on F. From G, it lands on C. From E♭, it lands on A♭.
Hear It — Song Associations for Ear Training
The fastest way to internalise the perfect fourth is to associate it with a tune you already know. Sing the first two notes of any of these and you have the interval.
The Interval in Chords
Every chord is a stack of intervals. Here is where the perfect fourth shows up in common harmony.
| Chord | Name | How P4 Appears |
|---|---|---|
| Csus4 | Suspended 4th | The 4th replaces the 3rd: C–F–G |
| C7sus4 | Dominant 7sus4 | Common in funk, jazz, and 1970s pop |
| Quartal voicings | Stacked fourths | Modern jazz piano voicings stack perfect fourths |
Inversion: Flip It Upside Down
When you move the bottom note up an octave (or the top note down an octave), the interval inverts. Two simple rules govern interval inversions:
- Numbers sum to 9. A 2nd inverts to a 7th, a 3rd inverts to a 6th, a 4th inverts to a 5th, and so on (1 + 8 = 9 for unison/octave).
- Quality flips. Major ↔ minor, augmented ↔ diminished, perfect stays perfect.
Compound Form
A compound interval is the same interval with an extra octave added on top. The character stays the same but the two notes are spread further apart. The compound form of the perfect fourth is the Perfect Eleventh (P11) — 17 semitones in total.
Why it matters: chord extensions like 9ths, 11ths, and 13ths are compound intervals stacked above the basic triad. A perfect 4th can be heard as a sus4 inside a triad; spread up an octave it becomes the 11th in jazz voicings.
Enharmonic Equivalents
Two intervals are enharmonic when they sound the same but are spelled differently. Same physical pitches, different musical meaning.
- Augmented Third (A3) — five semitones, rare; e.g. C–E♯
On the keyboard, an enharmonic pair sounds identical. On paper, the spelling tells you which scale or chord the note belongs to — and that changes how it functions in the music.
Frequently Asked Questions
Is a perfect fourth consonant or dissonant?
Both, depending on context. As a melodic interval or in quartal harmony it sounds open and stable. In traditional counterpoint, when the fourth appears above the bass, it is treated as a dissonance that needs to resolve.
What is the inversion of a perfect fourth?
A perfect fifth. P4 + P5 = 9, and quality stays perfect.
Why is it called "perfect"?
Historical convention. Intervals built on simple frequency ratios (1:1, 2:1, 3:2, 4:3) were considered "perfect" by medieval theorists because they sounded purely consonant.
Where do I hear perfect fourths in music?
In every "Here Comes the Bride" wedding, every fanfare, every quartal jazz chord, and at the start of "Amazing Grace."
How is the perfect fourth related to the perfect fifth?
They are inversions of each other. A perfect fifth from C up to G inverts to a perfect fourth from G up to C.