Perfect fourth
The perfect fourth spans five semitones with a just ratio of 4:3, an open interval whose stability depends on which note sits in the bass.
From C
- Size
- 5 semitones
- Abbreviation
- P4
- Just ratio
- 4:3 (498.04 cents)
- Equal-tempered
- 500 cents (+1.96 from just)
- Inversion
- Perfect fifth
What it sounds like
The perfect fourth is classed as a perfect consonance, but its role depends on context. Its ratio, 4:3, is simple, so the two notes sound clear and open. Melodically it is a bright upward leap, common in fanfares and hymn openings. Harmonically it is generally stable between two upper voices, but a fourth above the bass counts as a dissonance in tonal counterpoint and must be prepared and resolved. In a chord it is often heard as a suspension that wants to fall to a third.
How to recognise it by ear
The first two notes of 'Here Comes the Bride' rise by a perfect fourth. Without that tune, use C up to F in C major: the sound is open and lifting, with clear space above the lower note. It is the inversion of a fifth, so a listener who knows the fifth can find the fourth by moving the lower note of the pair up an octave. Practice: sing C up to F, then C up to G, one after the other, until the open fourth and the wider fifth are reliably told apart.
Theory
The perfect fourth inverts to the perfect fifth: the lower note moves up an octave, the semitones add to twelve, and both intervals remain perfect. In the major scale, the fourth degree lies a perfect fourth above the tonic. A sus4 chord, built from the root, fourth and fifth, contains the fourth directly. Its just ratio, 4:3, measures 498.04 cents. The equal-tempered perfect fourth is 500 cents, so the just size is 1.96 cents narrower.
The perfect fourth above every note
Spelled by letter name, so the interval’s number is always right: a perfect fourth above E♭ is not the same spelling as above D♯.
| From | To | Hear it |
|---|---|---|
On the piano
Further reading
- Reading Intervals at Sight: Generic and SpecificFundamentals · 6 min
- Parallel Fifths and Octaves: Why the Rule ExistsCounterpoint · 7 min
Questions
Why is the perfect fourth a consonance in some textbooks and a dissonance in others?
The interval itself does not change, but its role does. In tonal harmony, a fourth above the bass is treated as a dissonance, as in the 4-3 suspension, while a fourth between two upper voices is generally treated as consonant. Textbooks classify it by the role of the lower note, not by the ratio alone.
Are a fourth and a fifth inversions of each other?
Yes. Move the lower note of a fifth up an octave and the result is a fourth: C up to G becomes G up to C. The semitones add to twelve, and both intervals keep the perfect quality. The inversion swaps one perfect interval for the other, so a fourth is a fifth read in reverse.
Why does a fourth sound stable on its own but unsettled under a chord?
On its own, the fourth's simple 4:3 ratio sounds stable. Under a chord, the lowest sounding note takes the role of the root. A fourth above that bass sounds like a chord tone waiting to fall to the third, so it is heard as a suspension rather than a rest.