Fourth

Open and ambivalent

The perfect fourth is one of the oldest and most venerable consonances. In medieval organum from the 9th century it formed, together with the fifth and the octave, the basis of the first polyphony. Its harmonic role is ambivalent: in classical counterpoint the fourth between the bass and an upper voice counts as a dissonance that has to be resolved, although it sounds perfectly consonant between other voices.

In modern music the fourth has had 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.

The fourth on the guitar

Five frets apart: if you fret the high E string (329.63 Hz) at the 5th fret, A sounds at 440 Hz. The guitar is mostly tuned in fourths (E–A–D–G–B–E). Except for the G string, every string at the 5th fret has the same pitch as the next open string above. This is traditionally used for tuning. Between the G and B strings, however, lies a major third, four frets.

Fourth or fifth?

Both sound empty, and if you only pay attention to that, you are guessing. The remedy is to listen for the direction of the tension. The fourth floats and sounds open, the fifth feels stable. Two reference songs, one per interval, solve the problem permanently within two weeks.

Common mistakes

• Mixing up fourth and fifth: Five instead of seven semitones, and the tension points in a different direction.
• Taking the fourth for a dissonance: Between the bass and an upper voice it counts as a dissonance, between other voices as a consonance.
• Thinking of the guitar only in fourths: Between the G and B strings lies a major third.