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. By 6:30 p.m., the place was filled to capacity.

Had these people never seen a dueling-piano bar before? The Shout House, 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 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.

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

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.

After seven years, downtown's original dueling-piano bar is still going strong, inside Block E, no less. Shout House owner Joe Woods said he has a lease through 2014.

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.

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.

Jimmy Bernstein, the founder of Howl, 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.

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. 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.

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

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