Perfect 4th

General info

Interval

Perfect 4th

Quality

Perfect

Semitones

5

Degree distance

4 degrees

Degree

4

Interval structure

Lower note

Bb (466.2 Hz)

Lower note role

Root

Upper note

Eb (622.3 Hz)

Upper note role

Perfect fourth (4)

Degree formula

I

IV

Semitone formula

5 semitones (2.5 tones)

Interval construction

Interval formula

P4

Brief explanation

A perfect fourth spans five semitones and inverts to a perfect fifth. Its harmonic stability depends on its position above the bass and on style.

Interval notes

Consonance / dissonance

Sonority

Context-dependent interval

Explanation

In tonal counterpoint, a perfect fourth above the bass is normally treated as a dissonance; between upper voices it may belong to a consonant harmony. Modal and quartal textures treat its function differently.

How the interval sounds

Tendencies & resolutions

In a 4–3 suspension, the fourth above the bass resolves downward by step to a third. The same motion is common but not obligatory in an unprepared sus4 chord; a melodic fourth needs no independent resolution.

Harmonic context

A fourth is not a root interval in a standard triad, but it defines sus4 and serves as a building interval in quartal harmony. Most diatonic major and minor modes have a perfect scale degree 4; Lydian uses #4.

Interval inversion

Inverts to

Perfect 5thP5

Quality change

Perfect

perfect

Inversion rule

When inverting a perfect 4th, the lower note moves up an octave. 5 semitones (P4) become 7 semitones — a perfect 5th (P5).

Related intervals