Jazz is often difficult to define...

Jazz is often difficult to define, but improvisation is a key element of the form. Improvisation has been since early times an essential element in African and African-American music and is closely related to the use of call and response in West African and African-American cultural expression.

The form of improvisation has changed over time. Early folk blues music often was based around a call and response pattern, and improvisation would factor into the lyrics, the melody, or both. In the Dixieland style, musicians taking turns playing the melody while the others improvise countermelodies.

By the Swing era, big bands played using arranged sheet music, but individual soloists would perform improvised solos within these compositions. In bebop, however, the focus shifted from arranging to improvisation over the form; musicians paid less attention to the composed melody, or "head," which was played at the beginning and the end of the tune's performance.

As previously noted, later styles of jazz, such as modal jazz, abandoned the strict notion of a chord progression, allowing the individual musicians to improvise more freely within the context of a given scale or mode (e.g., the Miles Davis album Kind of Blue).

When a pianist, guitarist or other chord-playing instrumentalist improvises an accompaniment while a soloist is playing, it is called comping (a contraction of the word "accompanying"). "Vamping" is a mode of comping that is usually restricted to a few repeating chords or bars, as opposed to comping on the chord structure of the entire composition. Most often, vamping is used as a simple way to extend the very beginning or end of a piece, or to set up a segue.

In some modern jazz compositions where the underlying chords of the composition are particularly complex or fast moving, the composer or performer may create a set of "blowing changes," which is a simplfied set of chords better suited for comping and solo improvisation.