Since its development in the mid-1940s, bluegrass music has become one of the most distinctive American musical forms, attracting an intense audience of supporters who collectively form one of popular music's most vibrant subcultures. A close cousin of country music, bluegrass music is an acoustic musical style that features at its core banjo, mandolin, guitar, double bass, and fiddle along with close vocal harmonies, especially high-tenor harmony singing called the "high lonesome sound." Because of its largely acoustic nature, bluegrass is a term often used to describe all kinds of acoustic, noncommercial, "old-timey" music popular among rural people in the United States in the decades prior to World War II. That characterization, however, is incorrect. Bluegrass was developed, and has continued ever since, as a commercial musical form by professional recording and touring musicians. Often seen as being a throwback to this pre-World War II era, bluegrass...