Bluegrass is a style of southern string band music developed by Bill Monroe in the 1930s and 1940s. Bill Monroe's records with the Blue Grass Boys were so popular on the radio that announcers began using the term "Bluegrass music." Typical instrumentation is fiddle, banjo, mandolin, guitar, dobro, and bass.
Bluegrass comes from Mountain, Gospel, and Blues music, and generally involves a traditional, often melancholy, subject. It is played by an acoustic string band, which may be augmented by traditional instruments, such as the harmonica, mouth harp, accordion, jug, and spoons.
Bluegrass is characterized by virtuoso string playing, as with Bill Monroe playing the mandolin, or Earl Scruggs the banjo. The mandolin, following Monroe's example, and especially the 5-string banjo, is a prominent feature of the traditional bluegrass band. The tempo of the music is frequently fast, highlighting the skill of the performers; however, tempos and grooves vary from song to song, and tune to tune, and certain keys and modes are generally favored for "fiddle tunes" and songs, typically grounded in but not limited to blues and Celtic modal scales (Major, Mixolydian, Dorian and Aeolian). Some notable features of bluegrass are as follows: (1) musicians playing acoustic instruments in a continuously rotating ensemble, each "picker" taking turns playing lead "breaks" and "back-up" behind vocalists and other instrumentalists; (2) high-pitched -- often described as "high-lonesome" -- lead vocals and harmony singing in duos, trios and gospel quartets, often with bended notes and glissandos; (3) subject matter based on traditional themes akin to country and gospel music, of which bluegrass has many affinities and correspondences.