Blues is a music genre based on the use of the blues chord progressions and the blue notes. Though several blues forms exist, the 12-bar blues chord progressions are the most frequently encountered. Blue notes are sung or played at a slightly lower pitch than that of the major scale for expressive purposes. Blues emerged at the end of the 19th century as an accessible form of self-expression in African-American communities of the United States from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrative ballads. The use of blue notes and the prominence of call-and-response patterns in the music and lyrics are indicative of African influences. The blues influenced later American and Western popular music, as the blues form became a basic pattern of jazz, rhythm and blues, bluegrass and rock and roll. In the 1960s and 1970s, blues evolved into a hybrid form called blues rock. In the 1990s, punk blues appeared as an outgrowth of the blues rock and punk movements.