It's nearly 12 o'clock on a Saturday, and Jon Adelson knows what to play.

He taps the first few notes of Billy Joel's "The Piano Man" from his spot behind the keyboard at Mr. Smith's, the Georgetown piano lounge where he performs four nights a week. By the time he gets past the first line, his voice is nearly drowned out by the crowd, gathered around to sing.

"I look up from the piano, and I see everyone smiling," he said. "I say, 'There are worse things I could be doing.' "

It has been nearly two years since Adelson wandered into Mr. Smith's, asked whether they needed a piano player and earned himself a permanent spot. The son of a concert pianist, Adelson savors his perch as a fixture at the Georgetown bar, where he routinely pulls college students, professionals and out-of-towners from their drinks and bar stools to belt out lines to familiar songs.

The bar's nightly transformation from an unassuming M Street NW haunt with baseball on television to a music hall begins slowly. Adelson arrives about an hour before he performs to set up his keyboard and wooden facade made to look like the back of a piano. He spreads yellow "music menus" for patrons to use for requests.

Getting the audience involved is one of his trademarks, patrons and bartenders say.

"He's not up there to give a performance," bartender James Glover said as early patrons sang the chorus to "Sweet Home Alabama." Adelson's lack of ego keeps the atmosphere fun and allows everyone to sing, Glover said.

To Adelson, getting the crowd involved is part art, part science. He tries to get its pulse, often gauging the mood with an early test song -- something everyone knows, such as "Brown Eyed Girl." He determines the next songs based on the crowd's response; he says he never sticks to a prepared set.

No matter how quietly the night begins, something always changes, Adelson said. Maybe the alcohol kicks in or the crowd changes -- he's not sure. "I've never been able to define it."

Once it picks up, he turns to more classic singalongs, such as "American Pie." He encourages audience members and tries to adapt the key to complement their voices. If the crowd is really going, he said, he tries to roll from one song to another without a break.

Adelson, one of a few piano bar keyboardists in the city, attributed his musical proclivity to his mother, a concert pianist. He played piano, but by high school his attention had drifted to the guitar, which he calls "the instrument of the times."

After graduating from Tufts University in the 1960s -- he declined to say what year -- Adelson traveled the country performing at colleges and coffeehouses with a three-piece band that originally played folk and later turned to rock. He said he then settled in New York as a backup guitarist with Little Anthony and the Imperials and later began a stand-up guitar routine on the Upper East Side.

His musical career, though, took a back seat for 20 years when in 1976 he moved to San Diego, where he sold real estate. It paid the bills, he said, but wasn't thrilling.

When he reconnected with his college sweetheart, he said, he moved to Washington and returned to singalongs.

He had fallen in love with a sophomore the year he graduated from college, he said, and they dated through the summer. But he hadn't figured out his life and left for the band circuit. He married somebody else, but when they separated in San Diego, Adelson searched the Internet for Brett Williams, his college sweetheart and now a professor of anthropology at American University.

After an e-mail correspondence, the two arranged a meeting, and Adelson said he fell back in love. The rest came easily, he said.

"I'm a musician. She's a professor with five published books and tenure," he said. "So guess who moves?"

When he began at Mr. Smith's, Adelson worried that his repertoire would be wrong for younger patrons born after the days of "Mack the Knife." But they knew his music -- from Sinatra to the "Big Bopper," J.P. Richardson.

After nearly two years, Adelson has developed a crowd of regulars.

A group from Fort Myer used to be Wednesday night regulars until its members were deployed last year. He's still in touch with a few through e-mail and occasional phone calls.

Though he takes pride in his ability to get the crowd singing with him, Adelson takes his main pleasure from offering a chance to get away.

"I look at it as like the Billy Joel song -- 'to forget about life for a while,' " Adelson said. "I see all those smiling faces and say, 'That's cool.' "

Adelson described the job as much like the one described in "The Piano Man," which he called an uncannily accurate song. People sometimes repeat the last line to Adelson -- "Man, what are you doing here?"

"My answer is," he said, " 'I'm having the time of my life.' "

Jon Adelson performs at Mr. Smith's in Georgetown: "I look at it as like the Billy Joel song -- 'to forget about life for a while.' I see all those smiling faces and say, 'That's cool.' "