Octave
An octave spans twelve semitones. C up to the next C is an octave, and the upper note vibrates twice as fast as the lower one. The notes share a name because they belong to the same pitch class in different registers.
- 12 semitones
- C to C
- 2:1 frequency ratio
related pages
Hear it next to its neighbor
Compare this interval with the one a half step away, then play it going up, going down and together.
What is an octave?
Play middle C and then the next C to the right on a piano. You pass all twelve semitone steps and return to the letter C.
The upper note is clearly higher, yet it also sounds closely related to the lower one. Music notation reflects both facts: the letter name repeats, while an octave number or staff position identifies the register.
C4 and C5 are different pitches. They share the pitch class C. Pitch class ignores octave placement; pitch includes the exact register.
Why does frequency double at the octave?
Frequency measures how many times a vibration repeats each second. If A3 vibrates at 220 hertz, A4 vibrates at 440 hertz. A5 vibrates at 880 hertz.
Each step upward by an octave doubles frequency. Each step downward halves it. This produces the simple ratio 2:1 between the upper and lower notes.
The pattern is exponential. Moving from 220 to 440 hertz adds 220, while moving from 440 to 880 adds 440. Both jumps are one octave because each multiplies the frequency by two.
Why do the note names repeat?
Notes an octave apart share a strong perceptual relationship called octave equivalence. Listeners often hear them as versions of the same note at different heights.
The harmonic series helps explain the blend. If the lower note has a fundamental of 220 hertz, its second harmonic is 440 hertz, exactly the fundamental of the upper note. Many other harmonics also overlap.
Musical systems across cultures use octaves in many ways, but they do not all divide the space inside an octave into the same scales or assign it the same theoretical role. Western equal temperament divides it into twelve equal semitones, which is why this page counts twelve keyboard steps.
Why is it called an octave?
Count the letter names from C to the next C, including both ends:
C, D, E, F, G, A, B, C
That makes eight letter positions, so the interval is called an octave. The prefix oct- also appears in words connected with eight.
An octave belongs to the perfect interval family, along with the unison, fourth and fifth. Its full theory name is a perfect octave.
What is the difference between a unison and an octave?
A unison uses the same pitch in the same register. Two singers both singing C4 are in unison, even if their voices sound different.
An octave uses the same pitch class in a different register. One singer on C3 and another on C4 sing an octave apart.
A choir may sing one melody in several octaves. In everyday language people sometimes call that singing in unison because everyone follows the same tune. In interval terms, the individual voices are separated by octaves rather than true unisons.
How do octaves work in scales?
A one-octave C major scale begins on C and uses D, E, F, G, A and B before reaching C again. The second C closes the register span and begins the same scale pattern higher.
The scale contains seven different pitch classes. The eighth written note repeats the first pitch class, which is why scale diagrams commonly show eight note positions.
Scales can extend across several octaves, and some musical traditions organize pitch differently. An octave marks a repeating register relationship; it does not require every scale to contain seven notes.
How are octaves used in chords and arrangements?
Octave doubling repeats a chord tone in another register. C-E-G-C contains three pitch classes, so it remains a C major triad even though four piano keys are sounding.
Common uses include:
- A bass line doubled one octave higher for clarity and weight
- Piano left and right hands playing the same melody in different registers
- Guitar riffs built from the same note on two strings
- Orchestral instruments sharing a line in several octaves
- Synth patches combining oscillators one or two octaves apart
Doubling changes the texture, register and balance. It usually does not change the basic chord name because no new pitch class has been added.
How can a melody help you recognize it?
An ascending octave moves from a note to the next note with the same name, such as C4 to C5. A descending octave moves from C5 to C4.
Use the tune as a first memory cue, then practice the interval from different roots and in both directions. Also play both notes together so you learn its harmonic blend.
Why can two octave notes sound like one?
The 2:1 ratio aligns the upper fundamental with the lower note's second harmonic. Additional harmonics continue to overlap at regular points, so the combined spectrum contains many reinforcements.
This strong overlap can make the pitches fuse, especially when they share a similar timbre and start together. You can still hear a lower and upper register, but the pair often behaves like one enlarged note.
Real instruments complicate the ideal ratio. Piano strings are slightly inharmonic because stiffness pushes their upper partials sharp. Piano tuners therefore stretch high octaves slightly wide and low octaves slightly wide in the other direction so they blend better with the instrument's actual overtones.
A synthesizer using ideal harmonic waveforms can hold a precise 2:1 octave. The piano's stretched octave shows that tuning follows perception as well as arithmetic.