Compound Intervals: 9ths, 10ths & Beyond
Every interval bigger than an octave is really just a familiar interval wearing a bigger costume — a simple interval stacked on top of one extra octave.
Octave plus a simple interval
A compound interval is any interval larger than an octave. Add an octave to a 2nd and you get a 9th. Add an octave to a 3rd and you get a 10th. Add an octave to a 4th and you get an 11th, and to a 6th, a 13th. The math is simple: compound number = simple number + 7.
| Simple interval | + Octave | Compound interval |
|---|---|---|
| 2nd | +12 semitones | 9th |
| 3rd | +12 semitones | 10th |
| 4th | +12 semitones | 11th |
| 6th | +12 semitones | 13th |
Where you'll actually see them
Compound intervals show up constantly in jazz and modern harmony as chord extensions: a "C9" chord stacks a 9th above the root, a "C11" adds an 11th, and a "C13" adds a 13th. They're also the backbone of wide, open voicings across piano, guitar, and orchestral writing, since spreading notes across more than an octave gives a fuller, less cluttered sound than cramming them into one.
Common questions
What is a compound interval?
Any interval larger than an octave — a 9th is a 2nd plus an octave, a 10th a 3rd plus an octave, an 11th a 4th plus an octave, a 13th a 6th plus an octave.
Why do we call a 2nd-plus-octave a "9th" instead of a "2nd"?
The number counts the actual letter-distance spanned, including the extra octave — counting straight through gives 9 instead of 2.
Where are compound intervals used most?
Constantly in extended jazz chords (9th, 11th, 13th chords) and in wide, open voicings across many styles.
Does a compound interval sound different from its simple equivalent?
The underlying pitch relationship is identical, but the wider register genuinely changes the color — compound intervals sound more open and less clashing.
Hear extensions in real chords
Melody Monkey's Songwriting Studio lets you stack 9ths, 11ths, and 13ths and hear them instantly.
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