This is a pretty big question with lots of potential answers, but the answer you're probably looking for is the rules of harmony. https://en.wikipedia.org/wiki

This is a pretty big question with lots of potential answers, but the answer you're probably looking for is the rules of harmony. https://en.wikipedia.org/wiki/Harmony https://en.wikipedia.org/wiki/Harmony tl;dr: Frequencies that are related by some simple ratios are defined as "intervals" which are organized into scales. A handful of those make up the vast majority of popular western music and "sound good together".