When does resonance occur? Resonance between two physical media depends on the frequency ratio of their natural vibrations. When the two frequencies constitute a simple fraction such as 2/1 or 3/2, resonance can occur. In an earlier post, I explained how the ten simplest frequency ratios lead with mathematical necessity to the ten tones which appear in our musical scales, no matter whether they are major scales, the various minor scales, the ecclesiastical modes, the major pentatonic scales, the minor pentatonic scales, the blues scale, etc. Ten tones are sufficient to build all these different scales. Pure intonation and temperament