The perfect 4th and perfect 5th are the two most stable intervals after the octave: a perfect 4th spans five semitones and a perfect 5th spans seven. C to F is a perfect 4th and C to G is a perfect 5th, and the two are inversions of each other, adding up to an octave.
What is the difference between a perfect 4th and a perfect 5th?
The perfect 4th and the perfect 5th are the two most stable intervals in music after the octave, and they are closely related: they are inversions of each other, which means they add up to an octave. A perfect 4th spans five semitones and a perfect 5th spans seven, and five plus seven is twelve, the octave. C to F is a perfect 4th, five semitones, and C to G is a perfect 5th, seven semitones, and the two intervals are the same distance measured in opposite directions: F to C, going up, is a perfect 5th, while C to F, going up, is a perfect 4th. Both intervals sound open and stable, but they feel different. The perfect 5th sounds grounded and final, like a foundation, which is why it is the interval between the root and the 5th of every chord and between the tonic and the dominant of every key. The perfect 4th sounds slightly less settled, like a question waiting for an answer, which is why it is treated as a dissonance against the bass in classical harmony, even though it sounds consonant on its own. The 5th is the pillar of harmony; the 4th is the pillar's shadow.
How the 4th and 5th compare
Count them the same way as any interval: letters first, then semitones. D to G is four letters, D, E, F, G, and five semitones, so it is a perfect 4th. D to A is five letters, D, E, F#, G, A, and seven semitones, so it is a perfect 5th. The inversion relationship is easy to check: the 4th above D is G, and the 5th below D is also G, because the two intervals meet at the octave. The same relationship holds on every root, which is why learning one of the two intervals teaches you the other. The 4th and 5th appear in every chord and every scale. The 5th is the second note of every triad, the interval that gives the chord its stability, and the power chord is built from the root and the 5th alone. The 4th is the interval of the sus4 chord, where it replaces the 3rd, and it is the 4th degree of every scale, the note that gives the subdominant chord its name. The C major guitar chord contains the perfect 5th, C to G, and the Csus4 guitar chord replaces the 3rd with the perfect 4th, C to F.
Worked example — 4ths and 5ths above E. E to A is four letters, E, F#, G#, A, and five semitones, so it is a perfect 4th. E to B is five letters, E, F#, G#, A, B, and seven semitones, so it is a perfect 5th. Now invert them: A to E, going up, is five letters and seven semitones, a perfect 5th, and B to E, going up, is four letters and five semitones, a perfect 4th. The two intervals swap places when you turn them upside down, which is the inversion relationship in action. The E major guitar chord contains the perfect 5th, E to B, and the E major scale shows the 4th degree, A.
How to practise hearing the 4th and 5th
Practise the 4th and 5th as a pair, so your ear learns the difference between the grounded 5th and the questioning 4th.
- Play C and G together, a perfect 5th, then C and F together, a perfect 4th, and listen for the 5th sounding more settled than the 4th.
- Sing a perfect 5th above a root note, then sing a perfect 4th, and check yourself against the piano, aiming to feel the stability of each one.
- Play the open strings of a guitar in pairs, E to A, A to D, D to G, and name each interval: they are all perfect 4ths, five semitones apart.
- Play the C major scale and notice that the 4th degree, F, and the 5th degree, G, are the two notes that frame the middle of the scale.
- Use the interval calculator to quiz yourself on 4ths and 5ths from random roots until you can name each one from the sound alone.
Common mistakes
- Thinking the 4th is as stable as the 5th — The 4th sounds stable on its own, but against the bass it is treated as a dissonance in classical harmony, because it wants to resolve down to a 3rd. The 5th has no such restriction, and it is stable in every position.
- Confusing the 4th with the 11th — A 4th sits within the octave and an 11th sits above it, and they function differently in chords. A sus4 chord uses the 4th, while an 11th chord uses the 11th, and the two are not interchangeable.
- Counting the 5th as six semitones — A perfect 5th is seven semitones, not six: six semitones is a diminished 5th, the tritone. Counting one semitone short turns the most stable interval in music into the most unstable one.
Quick reference
This table lists the perfect 4th and perfect 5th above six roots, so you can see the two intervals side by side and check the inversion relationship in every row. In each row the 4th is five semitones above the root and the 5th is seven semitones above it, and the two add up to twelve semitones, the octave.
| Root | Perfect 4th (5 semitones) | Perfect 5th (7 semitones) |
|---|---|---|
| C | F | G |
| G | C | D |
| D | G | A |
| A | D | E |
| E | A | B |
| F | A# | C |
Play each row on piano, 4th first and 5th second, and listen for the 5th sounding more settled than the 4th. Then invert each interval: play the 4th above the root, then the same two notes in the opposite order, and hear the 4th become a 5th, which is the inversion relationship you can use to check your counting.
Frequently asked questions
What is a perfect 4th?
A perfect 4th is an interval of five semitones between two notes four letters apart, such as C to F or G to C. It is one of the four perfect intervals, and it sounds open and stable on its own.
What is a perfect 5th?
A perfect 5th is an interval of seven semitones between two notes five letters apart, such as C to G or D to A. It is the most important interval in harmony, after the octave, and it is the interval between the root and the 5th of every chord.
How are the 4th and 5th related?
They are inversions of each other: a perfect 4th up inverts to a perfect 5th down, and a perfect 5th up inverts to a perfect 4th down, because the two add up to an octave. Five semitones plus seven semitones is twelve, the octave.
Which is more stable, the 4th or the 5th?
The 5th, because it is the interval between the tonic and the dominant, and it sounds grounded in every position. The 4th sounds stable on its own but is treated as a dissonance against the bass in classical harmony.
Where do 4ths and 5ths appear in chords?
The 5th appears in every triad, as the interval between the root and the top note of the chord. The 4th appears in sus4 chords, where it replaces the 3rd, and in 11th chords, where it appears as a compound interval above the octave.