[rogerclark]

In order for it to fully make sense, you will need to study it from multiple angles. Hooktheory.com, books, and lots of YouTube videos will get you there over the course of 1-10 years. Ultimately, you won't understand music theory without trying to write a lot of your own music. It's like programming: you can read a book about JavaScript, but if you never wrote software and you never plan to, there's simply no way to actually understand the book.

An explanation of how this application might work (haven't verified from the source or letting it run long enough): let's say it chooses a subset of notes ("scale degrees") from a minor key, probably chosen at random. A subset of 5 or fewer notes from a scale (a pentatonic scale) will constrain the possible space of melodies so that most configurations will sound good. Even fewer and you get a more predictably pleasant (but perhaps less interesting) result. For instance, the notes that comprise the root chord will always sound good when played in any order. Acid lines typically suggest some kind of minor chord by playing an arpeggio (usually a repetitive melody consisting entirely of notes from a particular chord).

Also, a subset of notes from one scale are always going to be present in another scale. You can transition (modulate) from one scale to another by having a section use a shared subset of notes, then switching to the new one. In this manner, you can have a single piece of music traverse all possible scales (in acid techno, almost entirely minor scales).

[jacquesm]

> Ultimately, you won't understand music theory without trying to write a lot of your own music. It's like programming: you can read a book about JavaScript, but if you never wrote software and you never plan to, there's simply no way to actually understand the book.

That's a great observation, thank you!