While jazz may be difficult to define, improvisation is definitely one of its key elements. Early blues was commonly structured around a repetitive call-and-response pattern, a common element in the
African American oral tradition. A form of folk music which rose in part from work songs and field hollers of rural Blacks, early blues was also highly improvisational. These features are fundamental to the nature of jazz. Depending upon the performer's mood and personal experience, interactions with fellow musicians, or even members of the audience, a jazz musician/performer may alter melodies, harmonies or time signature at will.