Bluegrass music is a form of American roots music, and a related genre of country music. Influenced by the music of Appalachia, Bluegrass has mixed roots in Irish, Scottish and English traditional music, and was also later influenced by the music of African-Americans through incorporation of jazz elements.
Settlers from Britain and Ireland arrived in Appalachia during the 18th century, and brought with them the musical traditions of their homelands. These traditions consisted primarily of English and Scottish ballads—which were essentially unaccompanied narrative—and dance music, such as Irish reels, which were accompanied by a fiddle. Many older bluegrass songs come directly from the British Isles. Several Appalachian bluegrass ballads, such as "Pretty Saro", "Barbara Allen",
"Cuckoo Bird" and "House Carpenter", come from England and preserve the English ballad tradition both melodically and lyrically. Others, such as The Two Sisters, also come from England; however, the lyrics are about Ireland. Some bluegrass fiddle songs popular in Appalachia, such as "Leather Britches", and "Pretty Polly",
have Scottish roots. The dance tune Cumberland Gap may be derived from the tune that accompanies the Scottish ballad Bonnie George Campbell. Other songs have different names in different places; for instance in England there is an old ballad known as "A Brisk Young Sailor Courted Me", but exactly the same song in North American bluegrass is known as "I Wish My Baby Was Born".
In bluegrass, as in some forms of jazz, one or more instruments each takes its turn playing the melody and improvising around it, while the others perform accompaniment; this is especially typified in tunes called breakdowns. This contrasts with old-time music, in which all instruments play the melody together or one instrument carries the lead throughout while the others provide accompaniment. Breakdowns are often characterized by rapid tempos and unusual instrumental dexterity and sometimes by complex chord changes.
There are three major subgenres of bluegrass and one unofficial subgenre. Traditional bluegrass has musicians playing folk songs, tunes with traditional chord progressions, and using only acoustic instruments, with an example being Bill Monroe. Progressive bluegrass groups may use electric instruments and import songs from other genres, particularly rock & roll. Examples include Cadillac Sky and Bearfoot. "Bluegrass gospel" has emerged as a third subgenre, which uses Christian lyrics, soulful three- or four-part harmony singing, and sometimes the playing of instrumentals. A newer development in the bluegrass world is Neo-traditional bluegrass; exemplified by bands such as The Grascals and Mountain Heart, bands from this subgenre typically have more than one lead singer. Bluegrass music has attracted a diverse following worldwide. Bluegrass pioneer Bill Monroe characterized the genre as: "Scottish bagpipes and ole-time fiddlin'. It's Methodist and Holiness and Baptist. It's blues and jazz, and it has a high lonesome sound."
Unlike mainstream country music, bluegrass is traditionally played on acoustic stringed instruments. The fiddle, five-string banjo, guitar, mandolin, and upright bass (string bass) are often joined by the resonator guitar (also referred to as a Dobro) and (occasionally) harmonica or jaw harp. This instrumentation originated in rural dance bands and is the basis on which the earliest bluegrass bands were formed.
The guitar is now most commonly played with a style referred to as flatpicking, unlike the style of early bluegrass guitarists such as Lester Flatt, who used a thumb pick and finger pick. Banjo players often use the three-finger picking style made popular by banjoists such as Don Reno and Earl Scruggs. Fiddlers frequently play in thirds and fifths, producing a sound that is characteristic to the bluegrass style. Bassists almost always play pizzicato, occasionally adopting the "slap-style" to accentuate the beat. A bluegrass bass line is generally a rhythmic alternation between the tonic and dominant of each chord, with occasional walking bass excursions.
Instrumentation has been an ongoing topic of debate. Traditional bluegrass performers believe the "correct" instrumentation is that used by Bill Monroe's band, the Blue Grass Boys (mandolin, fiddle, guitar, banjo and bass). Departures from the traditional instrumentation have included dobro, accordion, harmonica, piano, autoharp, drums, electric guitar, and electric versions of other common bluegrass instruments, resulting in what has been referred to as "newgrass."