Grunge-country. Prog-bluegrass. Folk-punk. Hillbilly pop.

No matter what you name it, North Carolina picking-and-singing siblings Scott and Seth Avett have probably heard their music called it. And still you can't contain the Avett Brothers. Their rowdy, sold-out show at the Trocadero on Saturday proved that.

Since 2000, the Avetts have forged an aggressively played brand of Americana. With lyrics dedicated to passion and the consequences of mistakes, their rustic, impressionist arrangements give the ensemble a chamber-ragtime charm. Their records on the independent Ramseur label - ramshackle, mainly acoustic affairs - are messy to be sure, but contain the sort of silken vocal harmonies that could shame the Everlys.

Columbia Records head and über-producer Rick Rubin signed banjoist Scott (who plays bass drum while plucking), guitarist/pianist Seth, stand-up bassist Bob Crawford, and cellist Joe Kwon to his American label in the hopes of heightening the Avetts' popularity.

Yet with the rabidly enthusiastic crowd at the Troc - they knew every lyric, including the title track of I and Love and You, which won't be released until August - you couldn't imagine the quartet getting more popular.

Whether their songs were fully arranged ("Kick Drum Heart" and its contagiously melodic mix of art-pop and honky-tonk) or spare (a lonely "The Ballad of Love and Hate"), there was a rich abundance to the Avett sound. It was as if no single chord, soft-spun cello's saw, or rough falsetto yelp could hold their wealth.

There were breezily buoyant tracks with hollered-high bridges that came out of nowhere like "One Line Wonder" and heavy, rootsy, steal-your-woman tunes with descending-chord choruses like "Matrimony."

"Salina," off the band's 2007 album Emotionalism, became a winding suite that evolved from slow waltz to jiving jig to melancholic cello showcase, with a cinematic tone worthy of Wuthering Heights.

No matter what the melody or the genre, the heart of the Avetts' sound was the singular relationship between Seth and Scott, who swooned and cackled, swapped instruments like kids horsing around, and ended each other's phrases just like, well, brothers.