A perfect fourth is a musical interval spanning exactly 5 semitones—the distance between two notes where the upper note vibrates in a 4:3 frequency ratio to the lower note. This interval has a distinctive open, hollow quality that feels neither major nor minor. It’s less consonant than a perfect fifth but more open and spacious than a third, giving it a unique character in harmony.
Perfect fourths have been central to Western music for centuries, from medieval church chants to modern suspended chords and power chords.
How Many Semitones in a Perfect Fourth?
A perfect fourth is 5 semitones wide. Starting from any note and counting up the chromatic scale five steps will land you on the perfect fourth above.
For example, from C: C → C# → D → D# → E → F. That fifth step is F, which is a perfect fourth above C.
This 5-semitone distance is one semitone more than a major third (4 semitones) and one semitone less than a perfect fifth (6 semitones). Understanding these relationships helps you place the perfect fourth in the interval spectrum and makes ear training more efficient.
What Does a Perfect Fourth Sound Like?
The perfect fourth has an open, somewhat hollow or airy quality. It’s consonant enough to sound stable, but it lacks the warm sweetness of a major third or the resonance of a perfect fifth. Many musicians describe it as sounding “open” or “spacious”—it feels like there’s room between the notes rather than a blended harmony.
This open quality made the perfect fourth historically important. In medieval and Renaissance music, fourths and fifths were the primary intervals used in organum (early polyphony). Today, the perfect fourth appears in film scores, ambient music, and anywhere composers want that particular hollow, unresolved mood.
Perfect Fourths in Chord Construction
Perfect fourths form the backbone of suspended chords, often written as “sus4” chords. A Csus4 chord consists of C (root), F (perfect fourth), and G (perfect fifth). The sus4 voicing creates tension because the fourth replaces the expected major or minor third—so the chord sounds open and unresolved until it “resolves” back to the major or minor third.
Power chords, common in rock and heavy music, use a root, perfect fifth, and octave—no third at all. This construction gives power chords their neutral, aggressive quality without a clear major or minor tonality. The perfect fourth plays a complementary role in understanding how these chords stay open and edgy.
Understanding how notes combine into intervals and chords is essential for any musician or producer designing harmonic textures.
How to Recognize a Perfect Fourth by Ear
Start with a familiar reference. The opening of “Here Comes the Sun” by The Beatles begins with a perfect fourth (G–C). Use that or another song you know well as your anchor. Play the interval on an instrument and sing it back, paying attention to that open, hollow sensation rather than the blended sweetness of a major third.
The key difference: if you play a major third, it sounds complete and “happy.” A perfect fourth sounds open and needs resolution. Once you internalize that distinction, you’ll spot perfect fourths in melodies and harmonies quickly.
Practice with relative pitch exercises that isolate specific intervals. Sing the fourth, play it, and vary the starting note so your ear learns the interval relationship independent of absolute pitch. Pair your listening with playing—train both input and output simultaneously.
Perfect Fourth vs. Perfect Fifth
These two intervals are complementary. A perfect fifth spans 7 semitones; a perfect fourth spans 5. If you invert a perfect fourth (flip it upside down), you get a perfect fifth, and vice versa. A perfect fifth sounds rounder, fuller, and more resolved; a perfect fourth sounds more open and spacious. When you hear both intervals in close proximity—like in a sus4 chord—the contrast becomes obvious.
Understanding how frequency relationships determine interval character deepens your intuition about why these intervals sound the way they do.
Frequently Asked Questions
Is a perfect fourth always consonant?
Yes, perfect fourths are classified as perfect consonances in musical theory. However, they’re less blended than a perfect fifth. The 4:3 frequency ratio is a pure mathematical relationship, so the interval is consonant, but the open quality means it’s often used for contrast or suspension rather than stable resolution.
What’s the frequency ratio of a perfect fourth?
The frequency ratio is 4:3. If the lower note is 100 Hz, the upper note is approximately 133 Hz. This ratio is the same regardless of which notes you start from—it’s an interval relationship, not an absolute pitch.
Can a perfect fourth sound dissonant?
In context, yes. A perfect fourth used as a dissonance (like in sus4 chords) creates tension because it replaces an expected major or minor third. Out of context, the interval itself is consonant, but compositionally it can function as tension that resolves to a different interval.
How does a perfect fourth differ from a major third?
A perfect fourth is 5 semitones; a major third is 4 semitones. The major third sounds bright and warm; the perfect fourth sounds open and hollow. The major third is a fundamental building block of major chords; the perfect fourth is primarily used in sus chords for harmonic movement and tension.
Do all genres use perfect fourths the same way?
Not exactly. In classical and jazz, perfect fourths often function as suspensions that resolve downward. In rock and electronic music, fourths are used more freely for texture and atmosphere. In ambient and film scoring, the open quality of the fourth is valued for its own sake.
Vincent is a pitch detection and vocal analysis writer at OnlinePitchDetector. He focuses on pitch recognition, vocal frequency analysis, singing tools, and real-time audio testing for singers, musicians, producers, and beginners.