Ratios

Being a mathphobe, ratios have always been off-putting, but they're integral to music and music making, so they've got to be talked about somehow in the materials. The 2:3 ratio is both the hemiola in rhythm, the two against three, while in harmonic frequencies it's the Perfect Fifth, the same thing, just a lot faster. If one string is vibrating 200 times a second and another at 300, then we hear a fifth.