Blues is a music genre and musical form originated by African Americans in the Deep South of the United States around the end of the 19th century. The genre developed from roots in African musical traditions, African-American work songs, spirituals, and the folk music of white Americans of European heritage. Blues incorporated spirituals, work songs, field hollers, shouts, chants, and rhymed simple narrative ballads. The blues form, ubiquitous in jazz, rhythm and blues 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. Blue notes (or "worried notes"), usually thirds or fifths flattened in pitch, are also an essential part of the sound. Blues shuffles or walking bass reinforce the trance-like rhythm and form a repetitive effect known as the groove.