Generations in bluegrass
Bluegrass as a style developed during the mid 1940s. Because of war rationing, recording was limited during this time, and the best we can say is that bluegrass was not played before World War II, and it was being played after. As with any musical genre, no one person can claim to have "invented" it. Rather, bluegrass is an amalgam of old-time music, blues, ragtime and jazz. Nevertheless, bluegrass's beginnings can be traced to one band. Today Bill Monroe is referred to as the "founding father" of bluegrass music; the bluegrass style was named for his band, the Blue Grass Boys, formed in 1939. The 1945 addition of banjo player Earl Scruggs, who played with a three-finger roll originally developed by Snuffy Jenkins but now almost universally known as "Scruggs style", is pointed to as the key moment in the development of this genre. Monroe's 1945-48 band, which featured banjo player Earl Scruggs, singer/guitarist Lester Flatt, fiddler Chubby Wise and bassist Howard Watts, aka "Cedric Rainwater," created the definitive sound and instrumental configuration that remains a model to this day.
By some arguments, as long as the Blue Grass Boys were the only band playing this music, it was just their unique style; it could not be considered a musical genre until other bands began performing the same style. In 1947 the Stanley Brothers recorded the traditional song "Molly and Tenbrooks" in the Blue Grass Boys' style, and this could also be pointed to as the beginning of bluegrass as a genre.
It is important to note that bluegrass is not and never was a folk music under a strict definition; however, the topical and narrative themes of many bluegrass songs are highly reminiscent of "folk music". In fact many songs that are widely considered to be "bluegrass" are older works legitimately classified as "folk" or "old-time" performed in a "bluegrass" style. From its earliest days to today, bluegrass has been recorded and performed by professional musicians. Although amateur bluegrass musicians and trends such as "parking lot picking" are too important to be ignored, it is professional musicians who have set the direction of the genre. While bluegrass is not a folk music in the strictest sense, the interplay between bluegrass music and other folk forms has been studied. Folklorist Dr. Neil Rosenberg, for example, shows that most devoted bluegrass fans and musicians are familiar with traditional folk songs and old-time music and that these songs are often played at shows and festivals.
First generation bluegrass musicians dominated the genre from its beginnings in the mid-1940s through the mid-1960s. This group generally consists of those who were playing during the "Golden Age" in the 1950s, including Bill Monroe and his Blue Grass Boys, the Stanley Brothers, Lester Flatt & Earl Scruggs with the Foggy Mountain Boys, Reno and Smiley, the Lonesome Pine Fiddlers, Jim and Jesse, the Osborne Brothers, Mac Martin and the Dixie Travellers, and Jimmy Martin.
Bluegrass's second generation came to prominence in the mid- to late-1960s, although many of the second generation musicians were playing (often at young ages) in first generation bands prior to this. Among the most prominent second generation musicians are The Dillards, J. D. Crowe, Doyle Lawson, Sam Bush, John Hartford, Norman Blake, and Tony Rice. With the second generation came a growth in progressive bluegrass, as exemplified by second generation bands such as the The Country Gentlemen, New Grass Revival, Seldom Scene, and Del McCoury and the Dixie Pals. In that vein, first-generation bluegrass fiddler Vassar Clements, mandolin virtuoso David Grisman, Grateful Dead frontman Jerry Garcia (on banjo) and Peter Rowan as lead vocalist collaborated on the album Old and in the Way; the Garcia connection helped to expose progressive bluegrass to a rock music audience.
The third generation in bluegrass reached primacy in the mid-1980s. Third generation bluegrass saw a number of notable changes from the music played in previous years. In several regards, this generation saw a redefinition of "mainstream bluegrass." Increased availability of high-quality sound equipment led to each band member being miked independently, and a "wall of sound" style developed (exemplified by IIIrd Tyme Out and Lonesome River Band). Following the example set by Tony Rice, lead guitar playing became more common (and more elaborate). An electric bass became a generally, but not universally, accepted alternative to the traditional acoustic bass, though electrification of other instruments continued to meet resistance outside progressive circles. Nontraditional chord progressions also became more widely accepted. On the other hand, this generation saw a rennaissance of more traditional songs, played in the newer style.
 Fourth generation
It could be argued that a fourth generation of bluegrass musicians is beginning to appear, marked by a high level of technical skill. Although it is too soon to see definite trends, the most notable fourth generation musician to emerge so far is probably Chris Thile, who has recorded five solo albums since he was 13.
Since the late 1990s, several mainstream country musicians have recorded bluegrass albums. Ricky Skaggs, who began as a bluegrass musician and crossed over to mainstream country in the 1980s, returned to bluegrass in 1996, and since then has recorded several bluegrass albums and tours with his bluegrass band Kentucky Thunder. Around the same time, country music superstars Dolly Parton and Patty Loveless have both released several bluegrass albums. Along with the Coen Brothers' movie, O Brother, Where Art Thou? and the subsequent "Down From the Mountain" music tour, this has brought bluegrass music to a much wider audience. Meanwhile, bands such as the Yonder Mountain String Band in the United States, and Druhá Tráva in the Czech Republic have attracted large audiences while pushing at the edges of progressive bluegrass.
Though she is often considered a crossover or mainstream country artist, no discussion of recent developments in bluegrass music would be complete without mention of Alison Krauss. A vocalist/fiddler whose first album was released when she was just 16, Krauss and her band, Union Station, were major contributors to the soundtrack of 'O Brother, Where Art Thou?'. As a solo artist, collaborator, producer and with Union Station, Krauss has won, as of 2006, 20 Grammy Awards, the most of any female artist in history. She is now tied for 7th place on the all-time winners list.
Other notable recent bluegrass bands are the Earl Brothers, who write innovative music that remains evocative of bluegrass and old-time tradition, and Colorado's Open Road, a traditional-sounding band with original material.
In addition to what might be considered "mainstream" bluegrass, which has gradually changed over the last 60 years, two major subgenres have existed almost since the music's beginning.
Traditional bluegrass, as the name implies, emphasizes the traditional elements. Traditional bluegrass musicians are likely to play folk songs, songs with simple traditional chord progressions, and use only acoustic instruments. In the early years, traditional bluegrass sometimes included instruments no longer accepted in mainstream bluegrass, such as washboards, mouth harps, and harmonicas. Traditional bands may use bluegrass instruments in slightly different ways (claw-hammer style of banjo playing, or multiple guitars or fiddles within a band). In this sub-genre, the guitar rarely takes the lead (the notable exception being gospel songs), remaining a rhythm instrument. Melodies and lyrics tend to be simple, and a I-IV-V chord pattern is very common.
Main article: Progressive bluegrass.
The other major subgenre is progressive bluegrass, roughly synonymous with "newgrass" (the latter term is attributed to New Grass Revival member Ebo Walker). Progressive bluegrass came to widespread attention in the late 1960s and 1970s, as some groups began using electric instruments and importing songs from other genres (particularly rock & roll). However, progressive bluegrass can be traced back to one of the earliest bluegrass bands. A brief listen to the banjo and bass duets Earl Scruggs played even in the earliest days of the Foggy Mountain Boys give a hint of wild chord progressions to come. The four key distinguishing elements (not always all present) of progressive bluegrass are instrumentation (frequently including electric instruments, drums, piano, and more), songs imported (or styles imitated) from other genres, chord progressions, and lengthy "jam band"-style improvisation.