Fourths, fifths and octaves: the open sounds
Count and recognise the perfect fourth (5), perfect fifth (7) and octave (12), tell a fourth from a fifth with two well-known tunes, and see why a fourth up and a fifth down land on the same note.
THE SOLID ONES
Three distances that sound like stone
Thirds have a colour, bright or dark. The fourth, the fifth and the octave don't. Played together, their two notes blend so well that most people hear them as open, hollow and stable, as if there were one fuller note rather than two. They are the frame much of music is built on, and once you know them, you'll hear them everywhere.
Sing "Twinkle Twinkle" or "Here Comes the Bride" in your head. The very first leap in each is one of these three.
| Interval | Half steps | From C | Letters |
|---|---|---|---|
| Perfect fourth | 5 | C to F | C D E F (4) |
| Perfect fifth | 7 | C to G | C D E F G (5) |
| Octave | 12 | C4 to C5 | C to the next C (8) |
Check yourself
"Here Comes the Bride" starts on G and leaps up to the C above it on "comes". Hamid counts the half steps. What does he get?
- 4, a major third
- 7, a perfect fifth
- 5, a perfect fourth
- 6, because G to C skips over six keys
Show the answer
5, a perfect fourth
Right. G♯, A, A♯, B, C: five moves. And G A B C is four letters, so it's a fourth.
Your two reference tunes
Fifth up · 7
"Twinkle Twinkle": C C G G. The tune starts on its home note and leaps up to G on the second "twin-kle". Many people hear a fifth up as opening out, a wide, confident leap away from home.
Fourth up · 5
"Here Comes the Bride": G C C C. The tune starts below home and leaps up onto it. Many people hear a fourth up as arriving: the top note feels like where you were heading.
SAME NOTE, HIGHER
The octave: 12 half steps
Go up 12 half steps from C and you reach the next C. It has the same letter because it sounds like the same note, only higher. Played together, C4 and C5 blend so closely that most people hear one rich note. That's the octave's sound: the same, lifted.
When a man and a woman sing the same tune together, they are often an octave apart without thinking about it. It still sounds like one melody.
Check yourself
Reza plays C4 and C5 at the same time, and his sister says it sounds almost like a single note. That's what most people hear in an octave.
Show the answer
True
True. The two notes of an octave share the same letter and blend so closely that they often sound like one fuller note. That's why an octave is the easiest interval to mistake for no interval at all.
HOLLOW, NOT HAPPY OR SAD
Why a fifth sounds so bare
Play C and G together. There's no E or E♭, so there's no third, and the sound is neither bright nor dark. Most people call it open, hollow or strong. Rock guitarists use exactly this, root plus fifth, and call it a power chord. The fourth, C with F, sounds just as open and hollow, but many listeners hear it as a little less settled than the fifth.
Add E to C and G and the sound turns bright. Add E♭ instead and it turns darker. The fifth alone stays neutral.
Check yourself
Count up from the first note. Fourth, fifth or octave?
- C up to G
- G up to C
- A up to the next A
- D up to A
- C up to F
- E4 up to E5
Show the answer
Fourth · 5: G up to C, C up to F
Fifth · 7: C up to G, D up to A
Octave · 12: A up to the next A, E4 up to E5
FLIP IT OVER
A fourth up, a fifth down: same note
From C, go up to F: 5 half steps, a fourth. Now from the same C, go down to the F below: 7 half steps, a fifth. Same letter, two distances, and they add up to 12, one octave. Turning an interval upside down like this is called inversion. A fourth flips into a fifth, a fifth into a fourth.
It works the other way too. C up to G is 7, a fifth. C down to the G below is 5, a fourth.
Up from C4 to F4:
C♯ D D♯ E F
1 2 3 4 5 -> 5, perfect fourth
Down from C4 to F3:
B B♭ A A♭ G G♭ F
1 2 3 4 5 6 7 -> 7, perfect fifth5 + 7 = 12. Up and down together make one octave, F3 to F4.Any time you know one side, subtract it from 12 to get the other.
Check yourself
A song asks Sam to sing C and then the F above it, but that F is too high for his voice. He sings the F below the C instead. What interval is he singing now?
- Still a perfect fourth, because it's still C and F
- An octave, because he moved the F by an octave
- A perfect fifth down, 7 half steps
- A major third down, 4 half steps
Show the answer
A perfect fifth down, 7 half steps
Yes. C down to F is 7 half steps, a fifth. The fourth up flipped into a fifth down, and 5 + 7 = 12.
Check yourself
Match each sound to its interval
Show the answer
- "Here Comes the Bride": G up to C → Perfect fourth · 5
- "Twinkle Twinkle": C up to G → Perfect fifth · 7
- C4 and C5, like one richer note → Octave · 12
- C down to the F below → A fourth up, flipped
Lesson recap
- Perfect fourth = 5 (C to F), perfect fifth = 7 (C to G), octave = 12 (C4 to C5).
- Played together they sound open, hollow and stable; with no third, they are neither bright nor dark.
- Reference tunes: "Twinkle Twinkle" leaps up a fifth, "Here Comes the Bride" leaps up a fourth.
- An octave sounds like the same note, higher.
- Flip an interval and the two sizes add up to 12: a fourth up reaches the same note as a fifth down.