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The problem it solves
As you number and qualify intervals, you discover that the fourth, fifth and octave do not fit the major/minor pair. You need to know why they form a family of their own and what structural role they play.
Detailed theory
Key idea
Unison, 4th, 5th and 8th are perfect: they have no major or minor version, unlike seconds, thirds, sixths and sevenths.
From C: perfect 4th = 5 semitones (C–F), perfect 5th = 7 semitones (C–G), perfect 8th = 12 semitones (C–C’).
Understand it
Intervals fall into two families. Seconds, thirds, sixths and sevenths can be major or minor. But the unison, the fourth, the fifth and the octave form a family apart: the perfect intervals, which do not take the major/minor pair.
They are called perfect for their stability and acoustic purity. Their notes hold very simple frequency ratios, so they sound clean, solid and with little tension. That is why they hold up the bass, power chords and resolutions: they are the pillars harmony leans on.
From C, the perfect fourth (C–F) is five semitones, the perfect fifth (C–G) is seven and the perfect octave (C–C') is twelve. The unison, the same note twice, is zero semitones: the limiting case of the family. The perfect fifth is, after the unison and the octave, the most consonant interval there is, and the perfect fourth is its inversion.
An analogy: if the major or minor quality is the warm or cool colour of a wall, perfect intervals are the load-bearing beams of the building. You don’t argue whether a beam is happy or sad; what you value is that it holds. That is why we speak of stability, openness and foundation, not sadness or joy.
This solidity has practical consequences: the tuning of many instruments is based on chains of perfect fifths, and the rock power chord is nothing more than a perfect fifth with its octave. Recognising them gives you the foundation to build chords and understand why certain combinations sound so stable.
Staff & keyboard
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The three perfect pillars from C: the fourth (F), the fifth (G) and the octave (C'). Press each key to hear how the openness grows while always staying stable.
How to recognise it
How it's written
In English they are written with a P (for perfect) before the number: P4, P5, P8 (in Catalan and Spanish a J is used). On the staff the number is counted by the degrees it spans, just like any other interval; what changes is that here there is no major or minor version.
How it feels
Perfect intervals sound open, hollow and very stable, without the friction or warm colour of thirds and sixths. The perfect fifth has a wide, solid sonority (think of the sound of a power chord); the octave sounds like the same note repeated higher. Sing C–G to feel that clean openness.
Common mistake
Looking for whether a fourth, fifth or octave is major or minor: these intervals can only be perfect (or, when altered, augmented or diminished), never major or minor.
Confusing the perfect fourth (5 semitones, C–F) with the perfect fifth (7 semitones, C–G): they share the perfect family but have quite different sizes.
Try it
Play C–F (P4), C–G (P5) and C–C’ (P8) and notice the growing yet always stable openness of each interval.
From another note such as G, build the perfect fifth (G–D) and confirm it still sounds just as open and solid as C–G.
On the instrument
Interval distance
C–F: the perfect fourth, five semitones. It sounds open and stable; it is the inversion of the perfect fifth.
Interval distance
C–G: the perfect fifth, seven semitones. It is the most consonant interval after the unison and the octave; the heart of the power chord.
Interval distance
C–C': the perfect octave, twelve semitones. The same note one register higher, the peak of consonant purity.
Songs that start with each interval
- UnisonJingle Bells
- Perfect 4thHere Comes the Bride
- Perfect 5thBrilla, brilla (Twinkle Twinkle)
- OctaveOver the Rainbow
Well-known songs that start with each perfect interval: the opening of each melody lets you recognise the unison, the fourth, the fifth and the octave by ear.
The purity of just intervals
The purity of perfect intervals as waves: their frequencies hold very simple ratios (4:3, 3:2, 2:1), and it is precisely that simplicity that makes them sound clean and stable.
Where it's used
- Playing power chords
- Recognising that the rock power chord is a perfect fifth with its octave.
- Tuning by fifths
- Understanding why the tuning of many instruments is based on chains of perfect fifths.
- Holding up harmony
- Using perfect fourths, fifths and octaves to give a stable foundation to the bass and to resolutions.
Examples
Interval distance
G–D: a perfect fifth from another note. Seven semitones that sound just as open and solid as C–G.
Interval distance
F–Bb: a perfect fourth from F. Five semitones, the inversion of the fifth Bb–F.
Exercises
Recognise perfect intervals
Aural and visual recognition of intervals.
Complete 10 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/8 answeredQuestion 1/8
Which intervals form the perfect family?