Jazz is a unique kind of composition that, both follows the “rules” of classical music and ignores them. Jazz really has its own theoretical system that is generally more informal, but no less complicated, than classical music. There are many styles of “jazz” composition dating back to the first decade of the twentieth century. This section focuses on modern jazz, which begins around 1945. Like other sections in this book, the information presented here is merely an overview of select, key points.