Perfect fourth

Open and floating

With the ratio 4:3 the perfect fourth sounds open, floating and powerful. In medieval organum it formed, together with the fifth and the octave, the basis of the first polyphony. In classical counterpoint the fourth between bass and upper voice counts as a dissonance that must be resolved, although between other voices it sounds perfectly consonant.

The ratio 4:3 does not describe the partial at position 4 against the root (that would be 4:1, two octaves) but the distance from the third to the fourth partial: from the octave plus fifth to the double octave. Between them lies exactly one fourth.

Perfect fourth and sus4

In modern music the fourth has enjoyed a new flowering: sus4 chords such as Csus4 (C–F–G) use the fourth in place of the third and create an open, floating sound that can later resolve into a major or minor third. Sus4 chords therefore have no major or minor quality.

Guitar

The standard tuning E–A–D–G–B–E consists largely of fourths. The exception is the distance between the G and B strings, which is a major third. At the 5th fret every string except the G string matches the next higher open string, which is traditionally used for tuning.

Common mistakes

• Confusing the fourth with the fifth: Five against seven semitones.
• Hearing the fourth as an unambiguous consonance: Between bass and upper voice it counts as a dissonance.
• Thinking of the guitar only in fourths: Between the G and B strings lies a major third.