The bluegrass style was invented in the first half of the 20th century by mandolinist Bill Monroe and named for his band, the Blue Grass Boys, formed 1939. Monroe's 1945-48 band, which featured banjo player Earl Scruggs, singer/guitarist Lester Flatt, fiddler Chubby Wise and bassist Cedric Rainwater, created the definitive sound and instrumental configuration that remains the model to this day. Unlike mainstream country music, bluegrass continues to rely on acoustic stringed instruments. The fiddle, banjo, acoustic guitar or folk guitar, mandolin, and upright bass are sometimes joined by the dobro(also known as a resophonic guitar), and a bass guitar is occasionally substituted for the upright bass.
Besides instrumentation, the distinguishing characteristics of bluegrass include vocal harmonies featuring two, three, or four parts, often featuring a dissonant or modal sound in the highest voice; an emphasis on traditional songs, often with sentimental or religious themes; and improvised instrumental solos.