While many fans of bluegrass music date the genre back to 1939, when Monroe formed his first Blue Grass Boys band, most believe that the classic bluegrass sound jelled in 1946, shortly after Earl Scruggs, a 21 year old banjo player from North Carolina, joined the band. Scruggs played an innovative three-finger picking style on the banjo that energized enthusiastic audiences, and has since come to be called simply, "Scruggs style" banjo. Equally influential in the classic 1946 line-up of the Blue Grass Boys were Lester Flatt, from Sparta, Tenn. on guitar and lead vocals against Monroe's tenor; Chubby Wise, from Florida, on fiddle; and Howard Watts, also known by his comedian name, "Cedric Rainwater," on acoustic bass.

When first Earl Scruggs, and then Lester Flatt left Monroe's band and eventually formed their own group, The Foggy Mountain Boys, they decided to include the resophonic guitar, or Dobro into their band format. The Dobro is often included in bluegrass band formats today as a result. Burkett H. "Uncle Josh" Graves, from Tellico Plains, Tenn., heard Scruggs' three-finger style of picking in 1949 and adapted it to the then, almost obscure slide bar instrument. With Flatt & Scruggs from 1955-1969, Graves introduced his widely emulated, driving, bluesy style on the Dobro.

From 1948-1969, Flatt & Scruggs were a major force in introducing bluegrass music to America through national television, at major universities and coliseums, and at schoolhouse appearances in numerous towns. Scruggs wrote and recorded one of bluegrass music's most famous instrumentals, "Foggy Mountain Breakdown," which was used in the soundtrack for the film, Bonnie & Clyde. In 1969 he established an innovative solo career with his three sons as "The Earl Scruggs Revue." Scruggs still records and performs selected dates in groups that usually include his son, Randy on guitar, and his son, Gary on bass.

After parting with Scruggs in 1969, Lester Flatt continued successfully with his own group, "The Nashville Grass," performing steadily until shortly before his death in 1979.
By the 1950s, people began referring to this style of music as "bluegrass music." Bluegrass bands began forming all over the country and Bill Monroe became the acknowledged "Father of Bluegrass Music."