TonalityGUIDE - Tonal music basic theory for Undergraduates
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Harmonic series There are many theories about why the triad became so important in tonal music, but is is surely not a coincidence that the major triad (often regarded as the most stable) is constructed from the first three different notes of the harmonic series.

When you play a note on most instruments (try holding down the sustaining pedal on a piano and playing a loud C2) the sound you hear is made up of what acousticians call 'partials', 'overtones' or, more informally, 'harmonics'. The following example shows the overtones that sound when you play a low C: