Perfect fourth

What it means

The perfect fourth spans five semitones, its frequency ratio is 4:3. It is the complementary interval of the perfect fifth and closely related to it: the two together make up the octave. Its sound is open and empty, because it lacks the third, which would decide the mode.

Musical use

The dual nature of the fourth shapes its treatment. Between upper voices it counts as consonant and is unproblematic; standing above the bass, it is treated as dissonant in strict writing and demands preparation and resolution. This is why the six-four chord counts as needing clarification: it usually appears as a suspension before the dominant, where the fourth and sixth resolve stepwise downward into the third and fifth, or as a passing note — rarely as an independent sonority.

Equally common is the fourth suspension, in which the third of a chord is temporarily replaced by the fourth. The chord thereby remains undecided between major and minor until the resolution occurs.

Melodically the fourth is a forceful leap with a signal-like character; the falling fourth in the bass is the most common cadential movement of all. In twentieth-century music the fourth became the building block of a harmony of its own, in which chords are formed from stacked fourths instead of thirds.

When tuning guitars, most open strings lie at the distance of perfect fourths, on the double bass all of them. Many signal calls and hymn openings, too, begin with a rising fourth that marks an upbeat into the stressed beat. In harmony it is treated above the bass as a suspension that falls to the third. In the combined sound of two upper voices, by contrast, it seems stable and open.

Performance practice

Intonate fourths together with the fifths by the beats, before checking thirds. Resolve fourth suspensions distinctly and stepwise downward, and give the suspension weight beforehand — it is the actual event. Sing the fourth as an upbeat to a goal note; this way it is most securely hit.