Jazz is a genre of popular music that is highly rhythmic and in which improvisation plays a key role such that no two performances are ever identical. It was developed by African Americans in New Orleans in the U.S. in the late 19th and early 20th centuries and has been influenced by both African rhythms and European harmonic structures. Jazz contrasts particularly with classical music, which was developed in Europe, for which rhythm is usually much less pronounced, which is notated in great detail by the composer, and for which the emphasis is on performing it as closely as possible to the way the composer intended it.