Dueling-piano bars baffle me.

It was 6 degrees outside on a Friday night in late December. Even so, a line of people snaked out of the new downtown Minneapolis bar and stretched the length of the block. By 6:30 p.m., the place was filled to capacity.

Had these people never seen a dueling-piano bar before? Seems unlikely -- the Shout House, a mere three blocks away, has been packing 'em in most weekends since it opened seven years ago.

Ah, but this was opening night for Howl at the Moon, a so-called "rock 'n' roll dueling piano bar." Inside, a crowd squeezed around the stage, drinking buckets of booze (literally, 86-ounce buckets of neon-colored alcohol).

Led by the piano players onstage, the sold-out audience sang along to covers of Lady Gaga, LMFAO and, of course, Journey.

Gavin Steele, Howl's 24-year-old general manager, explained this mini-phenomenon. "We've got something for everybody," he said, as if he were describing a strip-mall buffet.

True enough, the crowd was a mish-mash of generations, and mostly women. "Bachelorette parties are huge with us," Steele said.

A new surge

Dueling-piano bars are nothing new. The concept saw a spike during the 1990s, when three bars opened in the Twin Cities, including one at the Mall of America.

In the past two decades, Howl at the Moon has become the country's largest piano-bar chain, with 15 operating along the Eastern Seaboard and throughout the Midwest. Here's how it works: Two piano players face off on a pair of baby grands. It's less a duel than a tag-team effort to rev up audience members who request songs and sing along like some mass audition for "American Idol." Crowd interaction is encouraged, with some songs leading to goofy comedy bits. Howl also has a drummer and guitarists.

The servers get in on the act, too. During one visit, I watched a cadre of waitresses jump up to sing "Lady Marmalade." After belting out Christina Aguilera's verse, Andrea O'Connor, 25, got off the stage and served three vodka-cranberries and a Miller Lite to a table down front.

"This job doesn't seem like a job," she said, screaming over the music.

The founder of Howl at the Moon is Jimmy Bernstein. The 62-year-old jet-setter opened his first Howl 21 years ago in Cincinnati. He no longer runs the day-to-day operations, but still oversees the concept.

"I have a very difficult assignment next week," he said by phone recently. "I have to go for a weeklong cruise to see how our Howl is doing on a Norwegian cruise line called the Epic."

Room for two?

Recently, on a relatively quiet Saturday night in downtown Minneapolis, I walked past the Shout House in Block E. The sounds of an old Billy Joel song emanated from inside. A group of guys brushed past me, one of them yelling out, "Dude, let's go to the Shout House!"

After seven years, downtown's original dueling-piano bar is still going strong -- very strong, when you consider that it's one of the last remaining tenants inside Block E. Shout House owner Joe Woods said he has a lease through 2014.

When Howl opened nearby in December, Shout House fans began shouting their allegiances on Twitter. Take this tweet from someone named Krista: "I don't think I could go to @HowlMinneapolis without feeling like I'm cheating on the @ShoutHouseMpls. I'm a one-piano-bar kinda girl."

Woods says his dueling-piano bar has stayed true to its concept since Day One. It's rock 'n' roll, but the sing-along aspect is key. "We're not a cover band," he said.

He wants people to interact with his musicians, to own the songs just as the pianists do.

"You hear a song and it takes you back to a place, whether it be high school or college or something later in life," Woods said. "When we perform a certain song in a particular way -- people don't forget stuff like that."

So many duels, so little time

Bogart's in Apple Valley, one of the Twin Cities area's longest-running nightclubs, just underwent a personality transplant. On Friday and Saturday nights, it's going by a new name: Double Ditty's Dueling Piano Party.

That's right, the suburbs once again want a piece of the dueling piano pie. Ltl Ditty's at the Mall of America closed in 2005. Other bars, like Axel's Bonfire and Jersey's, have tested the format, then ditched it. For years, Bogart's has been a hotbed for cover bands (it also houses a bowling alley). In December, the club completed an extensive remodeling, ditching the neon signs for a sleeker showroom-style venue.

The number of dueling-piano bars nationwide is at an all-time high, Bernstein said. The piano overlord said he's been involved in the opening of 19 Howl at the Moons. Not all have worked out. In the past few years, several have closed in cities such as Phoenix, Fort Lauderdale, Cleveland and Scottsdale.

He has a good feeling about Minnesota, though. The downtown location is a partnership with the Fortney brothers, who own the very successful Brothers Bar & Grill next door.

On that chilly -- but very busy -- opening night in December, Howl seemed to have already found its key clientele. At one point, as the piano players pounded out their version of Nicki Minaj's "Super Bass," a phalanx of women jumped onstage, dancing to the boom-badoom-boom-badoom beat.

Sitting off to the side were Susan Millar and Sue Motzko, both in their mid 40s. Sipping cocktails, they sang the praises of the piano-bar concept. They've been fans of the Shout House, they've visited clubs in other cities, they've even seen them on cruise ships (like Bernstein). The music takes them back to their youth. As do the 86-ounce buckets of booze.

"If I weren't driving, I'd drink it all myself," Millar said.

That's the kind of dedication a rock 'n' roll dueling- piano bar wants, I assume.

But is there room for more than one in the Twin Cities? By the looks of it, they should all do just fine. Shout House sold out its New Year's Eve show. Double Ditty's says it's bringing in national talent to play. And Howl at the Moon is the biggest bar to open in downtown in quite some time.

I'll just say it, so that no one else has to:

The age of the dueling-piano bar is upon us.