When the film O Brother, Where Art Thou? hit theaters two years ago, few could predict the smash success of its folk-and-bluegrass soundtrack. The album's quadruple-platinum sales so stunned the music business that even producer T-Bone Burnett struggled to explain. "This was real people playing and singing around real microphones," he said at the 2001 Grammys, where O Brother landed five awards. "It happened all at once and has exploded."

If that's the case, don't think it even singed the eyebrows of John McEuen. The silver-haired banjo wizard and his mates, the Nitty Gritty Dirt Band, bet their careers on the same formula 30 years ago - and beat the house. With its album Will The Circle Be Unbroken?, the first ever to gather the premier artists in country music, the Dirt Band found its roots, galvanized bluegrass and discovered a driving, melodious style that would make it the most durable country-rock outfit on the planet.

Proof positive will be on display tonight at the Ram's Head Tavern in Annapolis, where the Dirt Band - real folks around real mikes - will make its final stop on an 18-month world tour. Fans should get what the band has served up in venues from Sturgis to Switzerland: 2 1/2 hours of pop and country hits, blues and bluegrass virtuosity and lots of spontaneity.

"We do plan how we'll open a show," says McEuen. "We kind of plan how we'll finish, but that's just so we'll know when it's over. In between, anything can happen."

Those could be watchwords for the Dirt Band's sprawling career. The group started as a jug quintet in Southern California in the mid-'60s, when McEuen, a lanky teen, and fellow founders Jeff Hanna and Jimmie Fadden hung around McCabe's Guitar Shop, a Santa Monica folk institution. There they jammed, scarfed up the latest from Doc Watson and bluesman Brownie McGhee and generally studied, in McEuen's words, "how to make a living without getting a job."

The quest started small but promised a great deal. Jackson Browne was an early bandmate. McEuen took in shows by the Dillards, an influential bluegrass band then big in L.A. nightspots. Steve Martin, Duane and Gregg Allman and Chris Hillman, later of the Byrds, were friends. Fadden studied harmonica styles, Hanna researched folk artists from the 1930s, and the band dreamed up a sound they would pioneer along with the Eagles, the New Riders of the Purple Sage and the Flying Burrito Brothers: Country rock.

They picked a name that echoed what they'd draw on. "'Nitty Gritty' is a black folk phrase," says McEuen. "We thought it was funny: 'Let's get down to the nitty gritty.' Jeff and Jim said, 'We're not a blues band; we're not a bluegrass band. We're a dirt band.' The names came together. They stuck."

Over the next five years, so did NGDB. It put out five LPs, all of which showed off McEuen's precocity on fiddle, mandolin and banjo, Fadden's playful mouth harp and a vocal blend in which Hanna and Jimmy Ibbotson - who still swap leads on "Mr. Bojangles," the band's first hit - "sing like brothers," in McEuen's words.

The appeal was both American and off-the-wall. The band shared stages with Johnny Carson, Jack Benny and Bill Cosby - and with Aerosmith, Dizzy Gillespie and The Doors. "You can't say we haven't been around," says McEuen, chuckling.

In the late '70s, the Dirt Band became the first U.S. act to play the Soviet Union; in the mid-'80s, it played the L.A. Olympics and the first Farm Aid benefit, and in the early '90s, it toured Canada, Europe and Japan. Seventeen Top 10 Nashville hits made NGDB one of the top-selling acts in Music City history.

It was 1972, though, when the group set itself apart for good. It was touring when Bill McEuen, John's brother and the band's producer, suggested a stop in Nashville to make a record with some of the country artists the band admired. Those included Watson, an epic flatpicker little known outside folk circles; banjo rebel Earl Scruggs, and Mother Maybelle Carter, whose classics "Wildwood Flower" and "Keep On The Sunny Side" still define "Americana" music.

"It could have been career suicide," says McEuen with a laugh. "Here we were, a young band with a few hits, and we went to our record company and said, 'Hey, why don't we take some time and make music with a bunch of people you never heard of?' We wrapped the sessions in a week, but it didn't come out for a year. It was a risk."

The Dirt Band recorded its heroes live, as if they were playing around a campfire, and the resulting 38-song, three-LP set - complete with off-the-cuff storytelling - was so vividly human it unified "hippie" and "redneck" audiences. Wrote William Ruhlmann in The All-Music Guide: "The influence of [Will the Circle be Unbroken?] ... is incalculable. Maybelle Carter, Earl Scruggs, Doc Watson, Roy Acuff and others sat down with a bunch of longhairs, found common ground on the best of old-time country, and changed the direction of popular music."

The project went platinum, selling more than a million units by 1980. "I can't tell you how many people I meet who say, 'My dad used to play that record,'" McEuen says, "or, 'Hey, my brother turned me on to that stuff.' You know: 'The hardest thing about my divorce was you can't split three LPs.' It became part of people's lives." His guess: O Brother didn't create a new audience so much as found one waiting for a follow-up.

Circle fascinated a generation of new performers. Some, such as Vince Gill and Emmylou Harris, weighed in on Circle II (1989), which won three Grammys and the Country Music Association's album-of-the-year award. Modern stars Ricky Skaggs and Alison Krauss, Americana favorite Iris DeMent and country luminary Dwight Yoakam grew up listening to the collections. That's why McEuen, Hanna and Fadden chose them - along with Willie Nelson, Johnny Cash and Watson - to appear with NGDB on the latest installment, Will the Circle Be Unbroken?, Volume III. The band released that project as a CD - the band's 31st album - this month.

Changes along the way

Surveying NGDB's legacy makes McEuen feel like a history prof as he delineates its key eras.

In the early days, pre-1972, it drew on bluegrass. When current member Jimmy Ibbotson joined in 1969, he brought "a whole new energy," evident on one of NGDB's best records, Uncle Charlie and His Dog Teddy (1970). Keyboardist Bobby Carpenter came aboard in the late '70s, and his vocals "took the singing to the next level," says McEuen. The group moved to Nashville and staked its mainstream claim.

Many a band has perished for want of a clear identity, but the Dirt Band survived its eclectic nature in typical good humor. Merging country, folk, blues and rock "is like being in the country version of Spinal Tap," says McEuen. "We play a rock-and-roll show; they say, 'What's the country band doing here?' We play a bluegrass festival, and it's 'What's that rock-and-roll band doing here?' We play a country show, and it's 'Who let the bluegrass in?' The way I see it, it took 25 years for this band to find its viewpoint, for the common thread to be visible. It's all there now."

To McEuen, appreciating the Dirt Band is like viewing Impressionist art: It all depends on how close you stand.

"The Dirt Band is a pretty neat quilt of Americana music," he says. "If you're standing too near, you only see one square. ... But back up, and they're doing a Beatles song, bluegrass-style, or an original song they recorded with Johnny Cash. Or a little further back: 'Gee, that's something they did that was a pop staple' - like 'Mr. Bojangles.' It's a fortunate blend of talents that has been even more fortunate in its longevity."

The band got back from Switzerland a month ago. To McEuen's surprise, "That's really hot Dirt Band country. Over there ... you feel like you're in the Beatles. Three different people asked variations of the question, 'How have you stayed together so long?'

"First of all, we're the only ones we can work with," he says. "It's also due to fans of the music. They just keep coming! Other than that, there's a certain amount of constant change in the band, but it doesn't go too far away for our followers to latch onto."

Every gig's exciting

Continuity and change: that might be the theme of the Dirt Band's most lasting legacy, the Circle trilogy to which it gave birth.

A few giants have appeared on all three albums - fiddler Vassar Clements, crooner Jimmy Martin and Scruggs - but the cast has changed with time. The second album (1989) featured Gill, Skaggs and Emmylou Harris and won three Grammys. The third returns Watson for an encore with grandson Richard Watson. Circle III weaves in Cash, who contributes "Tears in the Holston River," a song he wrote for the late Maybelle Carter; Yoakam, who croons the Gram Parsons classic "Wheels"; Nelson, who teams with Tom Petty on "Goodnight Irene," and Krauss.

Ibbotson and Hanna chime in with originals. McEuen works out each of his instruments. Longtime friend Taj Mahal offers a spirited "Fishin' Blues," a song so homespun that even as a blues number it fits. "We wanted to do for Taj what we did for Doc on the first record," says McEuen. "Bring him to a Nitty Gritty audience."

As always, a dip back into tradition has energized the band. "This tour has been amazing," says McEuen. "So many people have said to me, 'I don't even like this kind of music - but I like this.' When we played Wolf Trap [in August], a guy in the crowd jumped up and hollered, 'This is the greatest night of my life!'

"I know what he means. For us, it has been like that every night. We can't wait to show up and play."

Concert

What: Nitty Gritty Dirt Band Reunion Tour

Where: Ram's Head Tavern, 33 West St., Annapolis

When: Tonight at 7 and 9:30

Tickets: $56.50

Call: 410-268-4545 (or online at www.ticketmaster.com)