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6.5.7 What Is a Harmonic Interval? Diatonic Intervals: Harmonic vs Melodic
To review for a moment, a melodic interval is the pitch distance between two different notes sounded successively. The term harmonic interval has two meanings. In one meaning, it's two different notes sounded simultaneously. In the other, used in this book, it's a chord progression. Diatonic harmonic intervals for a given key can only arise from triads built on roots belonging to the diatonic scale.
Why is this?
- The tonic note of a scale contains overtones that strongly reinforce scale degrees 1, 3 and 5. (Music always gets back to the brain recognizing simple-ratio overtones.)
- This in turn gives rise to the triad built on the tonic note, consisting of scale degrees 1, 3, and 5 of the diatonic scale, the overtones of which all reinforce each other internally.
- This gives rise to triads built on the other six notes of the diatonic scale.