Octave: the interval of twelve half steps
An octave is the interval between one note and the next note with the same letter name, 12 half steps higher or lower. The higher note vibrates exactly twice as fast, so A4 is 440 Hz and A5 is 880 Hz.
- Half steps12
- Short nameP8
- From CC to C
An octave is the distance between one note and the next note of the same name, higher or lower. It is the simplest and most natural interval in music, and the two notes sound so alike that they almost seem to be the same note at different heights. The name comes from the Latin word for eight, because you count eight letter names from one C to the next.
How big is an octave?
An octave is 12 half steps. Start on any piano key, count up twelve keys (black and white), and you arrive on a key with the same name. From C, the notes are C, D, E, F, G, A, B and then C again, which is eight letter names counting both ends. See piano keys for the full keyboard.
What is the science behind the octave?
Sound is vibration. The faster something vibrates, the higher the pitch. The number of vibrations per second is called frequency, measured in hertz (Hz). When one note is an octave above another, its frequency is exactly double.
| Note | Frequency (Hz) |
|---|---|
| A3 | 220 |
| A4 | 440 |
| A5 | 880 |
| A6 | 1760 |
The A above middle C, called A4, is usually tuned to 440 Hz, the standard orchestras and tuning devices use. One octave higher, A5 is 880 Hz. One octave lower, A3 is 220 Hz. Each octave up doubles the number, and each octave down halves it.
Why do notes an octave apart have the same name?
Because they sound so similar. When two notes are an octave apart, the waves line up closely, and our ears hear them as the same note in a higher or lower voice. This is called octave equivalence. It appears in many musical cultures around the world.
That is why the piano uses only seven letter names, A to G, and then repeats them. After G comes A again, one octave higher. Sharps and flats repeat as well. The pattern of white and black keys is the same in every octave.
How are octaves numbered?
To tell one C from another, musicians add a number, called the octave number. Middle C is C4. The C one octave up is C5, and the one below is C3. A new octave number begins on every C, so B3 is just below middle C, and C4 is the next key up. See middle C for more.
| Note name | What it is |
|---|---|
| C4 | Middle C |
| A4 | The A above middle C, 440 Hz |
| C5 | One octave above middle C |
| B3 | The B just below middle C |
A standard 88-key piano runs from A0 at the bottom to C8 at the top, a little over seven octaves.
What do 8va and 8vb mean?
Notes that go far above or below the staff need many ledger lines, which are hard to read. To avoid this, music uses octave signs. The marking 8va (from the Italian ottava, meaning octave) above a bracketed passage tells you to play it one octave higher than written. The marking 8vb (ottava bassa) below a passage means one octave lower. A dashed line shows how long the sign lasts.
There is also 15ma, meaning two octaves higher, which you will see less often. In piano music the left hand is sometimes told to play with octave doubling, with the same note in two octaves for a stronger bass.
Where do you hear octaves?
When men and women sing the same melody together, they usually sing an octave apart without thinking about it. A choir singing in unison often has voices in different octaves. On a guitar, the same note on different strings can be an octave apart. Many composers double a melody in octaves to make it sound bigger.
A well-known melody that opens with an octave leap is Somewhere Over the Rainbow. The first two notes jump up by an octave, so you can use that to remember the sound.
How can you practice octaves?
- Play any key on the piano, then count twelve keys up and play that note. Listen to how alike they sound.
- Find every C on the keyboard and name each one with its octave number.
- Sing a note and then sing it an octave higher or lower.
- Read a short passage with an 8va sign and play it an octave higher.
Memory trick: an octave doubles the frequency. A4 is 440 Hz, and each time you go up an octave you multiply by two. Going down you divide by two.
The octave is the first of the intervals covered at intervals. Its opposite in terms of tension is the tritone, which divides the octave in half.
Octave from every note
| From | perfect octave up | perfect octave down |
|---|---|---|
| C | C | C |
| C♯ | C♯ | C♯ |
| D♭ | D♭ | D♭ |
| D | D | D |
| E♭ | E♭ | E♭ |
| E | E | E |
| F | F | F |
| F♯ | F♯ | F♯ |
| G♭ | G♭ | G♭ |
| G | G | G |
| A♭ | A♭ | A♭ |
| A | A | A |
| B♭ | B♭ | B♭ |
| B | B | B |
Check yourself
1. How many half steps are in an octave?
Answer: 12. Counting twelve keys, black and white, from one note brings you to the same name an octave higher.
2. What is the frequency of A5 if A4 is 440 Hz?
Answer: 880 Hz. Moving up an octave doubles the frequency, so 440 Hz becomes 880 Hz.
3. Which note is middle C?
Answer: C4. Middle C is C4. A new octave number begins on each C.
4. What does an 8va sign above the staff tell you?
Answer: Play one octave higher. 8va means play the passage one octave higher than written.
5. Which interval is this?
Answer: Perfect octave. Two notes with the same name, 12 half steps apart, form a perfect octave.
Related pages
- Intervals in music: names, sizes and how to count them
- Tritone: the interval of six half steps
- Where is middle C on the piano and staff?
- Piano keys labeled: every note on the keyboard
- Ledger lines: notes above and below the staff
- The music staff: lines, spaces and clefs
- Piano notes: how the keys match the staff
- Half steps and whole steps (semitones and tones)
All intervals
See also: intervals explained.