Blues is a genre and musical form that originated in African-American communities in the "Deep South" of the United States around the end of the 19th century. The genre developed from roots in traditional African music, combined with European American folk music. Blues incorporated spirituals, work songs, field hollers, shouts, and 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. The blue notes (or "worried notes") which are often thirds or fifths which are flatter in pitch than in other music styles, are also an important part of the sound. Blues shuffles or walking bass reinforce the trance-like rhythm and form a repetitive effect called a groove.