The banjo is a stringed instrument with a thin membrane stretched over a frame or cavity to form a resonator. The membrane is typically circular, in modern forms usually made of plastic, originally of animal skin. Early forms of the instrument were fashioned by African Americans and had African antecedents. In the 19th century, interest in the instrument was spread across the United States and United Kingdom by traveling shows of the 19th century minstrel show fad, followed by mass-production and mail-order sales, including instruction method books. The inexpensive or home-made banjo remained part of rural folk culture, but 5-string and 4-string banjos also became popular for home parlor music entertainment, college music clubs, and early 20th century jazz bands. By the early 21st century, the banjo was most frequently associated with folk, bluegrass and country music, but was also used in some rock, pop and even hip-hop music. Among rock bands, the Eagles, Led Zeppelin, and the Grateful Dead have used the five-string banjo in some of their songs. Some famous pickers of the banjo are Ralph Stanley and Earl Scruggs.