Notes and intervals
Octave
An octave is the 12-semitone interval between two notes with the same name, the higher one vibrating twice as fast: on one guitar string it spans 12 frets.
What is an octave in music?
An octave separates two notes with the same name, like the open A string and the A at its 12th fret. The frequency doubles: the open A string sounds at 110 Hz, the 12th fret at 220 Hz. That is why the two notes blend almost into one.
Measured in semitones, an octave is 12. The name comes from the diatonic scale: counting from C you get C, D, E, F, G, A, B, C, and the second C is the eighth note. So an octave is both eight scale notes and twelve frets on one string.
How do you play an octave on guitar?
Three shapes cover the whole fretboard in standard tuning:
- Two strings up, two frets up: from the 6th to the 4th string, or from the 5th to the 3rd. Example: A at the 5th fret of the 6th string, A at the 7th fret of the 4th string.
- Two strings up, three frets up when you cross the B string: from the 4th to the 2nd, or the 3rd to the 1st. Example: open D on the 4th string, D at the 3rd fret of the 2nd string.
- Three strings up, three frets down: A at the 5th fret of the 6th string, A at the 2nd fret of the 3rd string.
The two E strings are two octaves apart, so the same fret on both gives the same note name: the 3rd fret of the 6th and 1st strings are both G.
Exercise: find every C on the neck
Start from one C and chain octave shapes to cover the neck:
- 5th string, 3rd fret
- 3rd string, 5th fret (two strings, two frets)
- 1st string, 8th fret (two strings, three frets, because you cross the B string)
- 6th string, 8th fret, then 4th string, 10th fret
- 2nd string, 1st fret
Repeat with G, then with F. Within a week you will place any root in seconds. In the guitar scales and chords tool, show a scale in note names to check every position of a given note.
Playing in octaves and doubling chord tones
Playing a melody in octaves thickens the sound without changing the harmony, the way Wes Montgomery did. Put your index finger on the 5th string and your ring finger two frets higher on the 3rd string, then mute the 4th string with the underside of your index. Play the C major scale: C (frets 3 and 5), D (5 and 7), E (7 and 9), F (8 and 10), strumming all three strings.
The octave also completes a power chord: A5 is the 5th fret of the 6th string, the 7th fret of the 5th string (the perfect fifth) and the 7th fret of the 4th string (the octave). Doubling a note never changes the chord's name.
Key takeaways
- An octave is 12 semitones: same note name, double the frequency.
- On one string it is 12 frets; across strings it is two strings and two frets (three when you cross the B string).
- Octave shapes are the fastest way to find a note anywhere on the neck.
- Doubling a note at the octave strengthens the sound without changing the chord.
Frequently asked questions
Why is the 12th fret an octave?
Each fret raises the pitch by one semitone and an octave has twelve. The 12th fret also sits exactly halfway along the string, which doubles the frequency. That is why the natural harmonic at the 12th fret matches the fretted note.
What is the difference between an octave and a unison?
In a unison both notes have exactly the same pitch: the 5th fret of the 6th string and the open 5th string are the same A. In an octave they share a name but one is twice as high: the 5th fret of the 6th string and the 7th fret of the 4th string.
Why does the octave shape change on the B string?
In standard tuning every pair of neighbouring strings is a perfect fourth apart, except G and B, which are a major third apart (one semitone less). Any shape that crosses that pair has to move up one fret to compensate.
Related terms
Practise in the workshop
Take it from theory to the fretboard
Open the workshop tools and see this idea across the whole neck. Free account, no card needed.