The ratio of 2A to A is 2:1, and we’ll be defining any two notes whose frequencies are in that 2:1 ratio as being one octave apart. And while those notes one octave apart are a pretty safe bet for composing, there’s only a handful of them in human hearing range, and they get boring pretty quick. Other generally-accepted-as-pleasant-across-cultures ratios for notes include 3:2, 4:3, 5:4, and (somewhat more contentiously) 5:3, 6:5, and 8:5. But you don’t want to only have consonance in your songs: introducing, building up, and resolving dissonance can help give a song surprising beauty and
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