Jazz is a performance style that grew up among black musicians around 1910 and has since gone through a series of extraordinary developments. Its first key feature is improvisation. When jazz musicians play a song, they do not stick to a written score or duplicate the way they have heard it before. Instead they freely elaborate around a song. They add ornaments and newly contrived interludes, called breaks. In effect, they are always making up variations on the tunes they are using — variations sometimes of such complexity that the original song almost disappears.

The second key feature of jazz is a special rhythmic style involving highly developed syncopation. Syncopation occurs when some of the accents in music are moved away from the main beats, the beats that are normally accented (see page 7). For example, in 2/2 meter, instead of the normal ONE two ONE two, the accent can be displaced from beat 1 to beat 2 — one TWO one TWO. This is called a “backbeat” in jazz parlance.

In addition, jazz developed syncopation of a more subtle kind, sometimes called beat syncopation. Derived from African drumming (see page 397), this technique can also be traced in earlier black American music. In beat syncopation, accents are moved just a fraction of a beat ahead of the metrical points. When this happens in just the right way, the music is said to “swing.”

Notice that jazz is not so much a kind of music — the music it is based on usually consists of popular songs, blues, or abstract chord series called “changes” — but a special, highly charged way of performing that music.