Bluegrass has its roots in the folk music of the American states of West Virginia, Virgina and Kentucky (nicknamed 'The Bluegrass State'). It is the music of the poor white farmers who were called 'hillbillies' in the city. The characteristic line-up of a bluegrass combo consists of guitar, mandolin, violin and often also a double bass. Also striking are the beautiful harmonic vocals of the band members. Bill Monroe was the first in the fifties to call his music 'bluegrass'. The genre later experienced a number of striking revivals, partly due to the success of the film O Brother Where Art Thou? in which bluegrass sounds. Violinist and singer Alison Krauss is considered the greatest bluegrass artist of the twenty-first century.