Interval reference
Perfect octave
8 letter names · 12 semitones · C → C
A perfect octave spans eight letter names and 12 semitones. Both halves of that description matter: the number comes from the letters, the quality from the exact size.
Starting on C, a perfect octave reaches C. Its shorthand is P8, and traditionally it counts as perfect consonance.
How big it is
Turned upside down
Move the lower note up an octave and a perfect octave becomes a perfect unison. The two numbers add up to nine, and the quality flips — major with minor, augmented with diminished, while perfect stays perfect.
Where you meet it
- The same note, higher
- Octave doubling between instruments
From every starting note
| From | Up a perfect octave |
|---|---|
| C | C |
| C♯ | C♯ |
| D | D |
| D♯ | D♯ |
| E | E |
| F | F |
| F♯ | F♯ |
| G | G |
| G♯ | G♯ |
| A | A |
| A♯ | A♯ |
| B | B |