Blues is a music genre and musical form which was originated in the Deep South of the United States around the 1870s by African-Americans from roots in African musical traditions, African-American work songs, and spirituals. 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, fifths or sevenths 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.