On the Black Keys

o What: Black Keys, in concert.

o When: 8 p.m. Oct. 4-5.

o Where: McMenamins Crystal Ballroom, 1332 W. Burnside St., Portland.

o Cost: $36.80 through Ticketmaster, 800-745-3000 or http://ticketmaster.com.

o Information: 503-225-0047 or http://danceonair.com.

Fans who see the Black Keys on tour this fall will see something very different from the familiar group. During a key segment of the show, it won’t be just Dan Auerbach on guitar and vocals and Patrick Carney on drums — the way it’s been for the Black Keys live since the band formed in 2001.

“We’re going to be playing a 30-minute block off of the new record with a bass player and a keyboard player,” Carney revealed in a recent phone interview. “So we’re going to start the show off as a two-piece, do a middle section with a four-piece and then end as a two-piece.”

The evolution of the Black Keys into a four-piece band — at least for part of its live show — is a symbol of the major step the duo has taken with its music on “Brothers.”

“I think maybe for a long time there was an understanding, even though it was mostly unspoken, that we were a guitar-and-drums combo,” Carney said. “ And when we did (the 2008 CD) ‘Attack & Release,’ we were consciously trying to make a record that was not really sticking to that formula, but we were still heading into that with the understanding that we’d have to play these songs live as a two-piece. I think when we went in to make this record, we never even considered how we were going to re-create it (live).”

Chances are, fans will like what they hear on “Brothers,” despite the fact that the album stands as a departure at times from the duo’s past work.

The new album finds the Black Keys boldly stretching out on songs like the deliciously funky “Everlasting Light.” “Tighten Up” takes a more soulful direction, while delivering the rhythmic wallop Black Keys fans have come to know and love.

Meanwhile, other songs (such as “She’s Long Gone,” “Next Girl” and “Howlin’ For You”) hew closer to the Black Keys’ familiar gritty bluesy rock sound.

In essence, Auerbach and Carney have managed the tricky task of expanding their sound without losing the stylistic identity of the Black Keys.

That identity was solidly established by the group’s first three albums, “The Big Come Up” (2002), “Thickfreakness” (2003) and “Rubber Factory” (2004). Those releases featured a sound built almost exclusively by electric guitar and drums and rooted in rough-and-tumble blues and riff-heavy garage rock.

Then on “Attack & Release” came signs that Auerbach and Carney were starting to think in broader terms musically, particularly when the group showed a wider range of influences with the hill-country tones of “Psychotic Girl,” the acoustic country blues of “All You Ever Wanted” and the ambient tones of “Remember When.”

Carney feels the musical evolution of “Brothers” is also partly an outgrowth of outside projects he and Auerbach undertook in 2009.

Auerbach was the first to surface, releasing a solo album, “Keep It Hid,” in February 2009. Later in the year, Carney released the self-titled debut by his side band, Drummer — an album that took a notably pop-filled turn away from the Black Keys.

The two then united for a project called “Blackroc,” in which Auerbach and Carney collaborated with a variety of hip-hop artists (Mos Def, RZA, Raekwon, to name a few) that brought together the worlds of rock, hip-hop and R&B. That project in particular, Carney said, shaped the “Brothers” CD.

“When we made ‘Blackroc,’ we were really kind of forced to not think in terms of sounding like the Black Keys,” he said. “We were starting every song with bass and drums. That’s how we started every song on ‘Brothers.’ Also when we were working on the Blackroc stuff, watching the MCs pick the tracks they wanted to rap on, it always seemed like they would gravitate more toward the more mellow kind of back tracks. I think that maybe there’s something to that. Maybe there’s more of a universal appeal to, like, a steadier groove than something more frantic.”

The final piece of the puzzle came together when Auerbach and Carney had Tchad Blake mix “Brothers.”

“I think that his mixes brought out the grit that was intended to be there,” Carney said. “Tchad is a master of style. He can mix Sheryl Crow’s record and he can mix Tom Waits and the Black Keys and everything is going to sound really good and everything is going to sound really different.”