Noted jazz critic Joachim Berendt defines jazz as: "A form of music which originated in the United States through the confrontation of African Americans with European music. Jazz differs from European music in that jazz has a special relationship with time, defined as "swing"; a spontaneity and vitality of musical production in which improvisation plays a key role; and a sonority and a manner of phrasing which mirror the individuality of the performing jazz musicians." While jazz music may be difficult to define, improvisation is clearly one of it's key elements. Hence, jazz musicians develop a special kind of relationship, always placing emphasis on interaction with each other. In jazz, the skilled performer will interpret a tune in very individual ways, never playing the same composition exactly the same way twice. Although post WWII jazz music has gone through many changes over the years: bebop, post-bebop, cool jazz, third stream, Latin/Afro Cuban, modal, avante guard, soul jazz, jazz fusion, jazz funk, smooth jazz and acid jazz- the main stay of modern jazz still remains the quartet of saxophone or trumpet, piano, bass and drums.