Fourth
Interval of five semitones.
What it means
The fourth spans four scale degrees; in its perfect form that is five semitones, with the frequency ratio 4:3. It is the complementary interval of the fifth — the two together make up the octave. Alongside the perfect fourth there is the augmented one with six semitones, which corresponds to the tritone, and the diminished one, which tonally coincides with the major third.
Musical use
The fourth occupies a peculiar position in part-writing theory. Between two upper voices it counts as consonant; above the bass, by contrast, it is treated as dissonant and demands preparation and resolution. This explains why the six-four chord — the second inversion of the triad, with the fifth in the bass — counts as needing clarification and usually stands as a suspension before the dominant rather than as an independent sonority.
Melodically the fourth is a forceful, well-singable leap with a signal-like effect; countless fanfares, chorale openings, and call motifs begin with it. The falling fourth in the bass is at the same time the most common cadential movement — from the dominant to the tonic.
In twentieth-century music the fourth became the building block of a sound world of its own: in quartal harmony, chords are formed from stacked fourths that elude third-based order and sound more open, more indeterminate.
The fourth is the inversion interval of the fifth and arises between the third and fourth partials of the harmonic series. Above the bass it counts, in traditional part-writing, as a dissonance, between upper voices as a consonance; hence the special position of the six-four chord. As a melodic leap, the ascending fourth is a frequent beginning of songs and hymns. In twentieth-century music and in jazz it serves as the building block of quartal chords.
Performance practice
Sing the fourth as an upbeat to a goal note — in this shape it is especially easy to hit. Intonate the perfect fourth by the disappearance of the beats, together with the fifth, before checking thirds. And do not treat the six-four chord as an incidental passing sonority; it demands a continuation.