A genre and musical form developed by African Americans in the United States around the end of the 19th century from African-American work songs and European-American folk music. The blues form, ubiquitous in jazz and rock and roll, is characterized by the call-and-response pattern, the blues scale and specific chord progressions, of which the twelve-bar blues is the most common. Blues shuffles or walking bass reinforce the trance-like rhythm and form a repetitive groove effect.