Harmony is a matter of thirds, fifths, sevenths, and octaves
Okay. If you’ve read that heading and you’re not a musician, you might be wondering what on earth it could possibly mean. Let’s make it easy: harmony describes the relationships between notes, and harmonisation refers to the process whereby certain notes sound pleasing together (such as the C, E, and G that make up a C Major chord).
It won’t come as a surprise that harmonisation is as mathematical as it is musical. Indeed, musicians talk about thirds, fifths, sevenths and so on because these terms describe ratios (there’s that term again) that occur between notes that have a sympathetic relationship with each other. There’s an elaborate science devoted to harmony, so look it up if you want to take a deep dive. For now, simply know that, without harmonic ratios, your favourite songs would sound far, far worse!