When the improvisation takes place, the basic elements of the head continue (a sequence of chord progressions, bass lines, etc.) but now the melodic line is replaced with an improvised one. This section is often of indeterminate length but generally lasts as long as it takes for each musician to make an improvisation over one pass of the composed tune.

Listen to this excerpt from a jazz performance, and while listening, try to identify elements that help these two superb musicians, vocalist and bassist, create a rich dialogue between them. ("Do Nothing Till You Hear From Me," music by Duke Ellington, lyrics by Bob Russell. Performed by Ella Fitzgerald, voice.)