Perfect Fourth

Play C followed by F. The leap is wider than a third and one semitone narrower than a tritone. It often sounds broad and direct in a melody.

Now play C and F together. The pair can sound open on its own, especially in modern music. Keep C in the bass, add G and then let F fall to E. The notes change from Csus4, C-F-G, to C major, C-E-G. That one-semitone resolution reveals why the fourth can also carry tension.

The sound of the interval does not change its size. Its role changes because the bass and surrounding chord give F somewhere to go.

Count the letter names from C to F, including both ends: C, D, E, F. That makes it a fourth. The five-semitone distance makes it perfect.

C to F sharp still spans four letter names, but it covers six semitones. That is an augmented fourth and one spelling of the tritone.

C to F flat spans four letter names and four semitones, so it is a diminished fourth. It sounds like C to E on an equal-tempered piano, but its spelling shows a different scale or voice-leading function.

C up to F is a perfect fourth. Move the lower C above F and the interval becomes F up to C, a perfect fifth.

The two intervals are inversions. Their semitone counts add to twelve: five for the fourth and seven for the fifth. Their interval numbers add to nine: four plus five.

This is why the intervals often feel related. A two-note pair such as C and F can be heard as a fourth when C is below, or as part of an F-based root-fifth relationship when F takes the harmonic role of root. The bass and musical context help your ear choose an interpretation.

In modern pop, folk and jazz, a fourth can be treated as an open sonority. A stack such as C-F-B flat can remain in place as quartal harmony without needing to turn into a conventional triad.

In common-practice tonal harmony, a perfect fourth directly above the bass is often treated as a dissonance. Over a C bass, F may resolve down to E, changing the fourth C-F into the third C-E.

This is especially clear in a 4-3 suspension. The F is held while the bass changes or the harmony moves underneath it, creating a fourth above C. F then falls by one semitone to E.

The apparent disagreement about whether a fourth is consonant or dissonant comes from these different contexts. The interval alone gives you the distance; the bass, meter and voice leading tell you its job.

A Csus4 chord contains C, F and G. F replaces the third E, so the chord does not state C major or C minor in the usual way.

When F moves down to E, Csus4 becomes C major. That familiar motion gives the suspended fourth its sense of release. The resolution is common, but modern music can also keep the sus4 chord unresolved and use it as a stable color.

A Cadd11 chord is different. It keeps the third E and adds F, usually in a higher register. E and F form a minor second, so arrangers often separate the notes or omit E when they want a cleaner voicing.

A C major triad is C-E-G. Put G in the bass and arrange the other notes above it as G-C-E. C sits a fourth above the bass and E sits a sixth above it, so classical theory calls this a six-four chord.

Although the pitch classes still spell C major, the bass changes how the chord behaves. In a cadential six-four, the fourth and sixth above the dominant bass commonly resolve to a third and fifth before the harmony moves to the tonic.

For example, over a G bass, C can fall to B and E can fall to D. The resulting notes G-B-D form G major or part of G7. The C-major notes function as accented tension over G rather than as an independent tonic chord in root position.

The fourth degree of major and natural minor scales forms a perfect fourth above the root. In C major and C minor, that note is F.

Most diatonic modes also use a perfect fourth above the root. Lydian is the exception: C lydian contains F sharp, an augmented fourth above C. That raised fourth is the note that separates lydian most clearly from the major scale.

Common harmonic uses include:

A pure perfect fourth has a frequency ratio of 4:3. If the lower note vibrates 300 times per second, the upper note vibrates 400 times. Their wave patterns line up regularly, which supports the interval's strong blend.

A pure fourth measures about 498 cents. A piano in equal temperament uses exactly 500 cents, only about two cents wider. The difference is subtle in most playing but can matter when singers or string players sustain exposed notes and adjust their tuning by ear.

The fourth and fifth are also tuning partners. A pure 4:3 fourth is the inversion of a pure 3:2 fifth. Equal temperament shifts each by about two cents in opposite directions so that all twelve keys fit the same fixed keyboard.