What is klezmer music? Before long, it may be easier to ask what it isn't.

Bogdan Krezel Clarinetist David Krakauer will play at 7:30 p.m. Monday at Rodef Shalom Congregation. Click photo for larger image. Pittsburgh Jewish Music Festival: Klezfest '07 Brave Old World, "Song of the Lodz Ghetto." 7:30 p.m. tonight. Jewish Community Center, Squirrel Hill. Steel City Klezmorim, 7:30 p.m. Wednesday. Club Cafe, South Side. David Krakauer, clarinetist, in concert with members of the Pittsburgh Symphony and pianist Luz Manriquez. Monday 7:30 p.m. Rodef Shalom Congregation. Tickets: $20 ($15 seniors, $10 students); 412-394-3353. Listen In: Hear examples of Klezmer music: "S'iz kaydankes, kaytn" from Brave Old World's song cycle about the holocaust, "Song of the Lodz Ghetto." Clarinetist David Krakauer and the Kronos Quartet perform Osvaldo Golijov's "The Dreams and Prayers of Isaac the Blind." Hip-hop artist SoCalled joins Krakauer for "Moskovitz and Loops of It." The Steel City Klezmorim, from an audio clip on its Website, with Henry Shapiro here on bass.

A centuries-old genre, klezmer is now melding with all manner of contemporary music and instruments. This year's Pittsburgh Jewish Music Festival will look at klezmer's past and its ongoing revival by welcoming three klezmer bands: Brave Old World, Steel City Klezmorim and clarinetist David Krakauer and friends. What these groups offer may surprise anyone prone to strict definitions.

"The working definition of klezmer music is Eastern European Jewish celebration music -- wedding, dance and party music," says Krakauer, a clarinetist and one of the world's eminent performers of this undeniably energetic and infectious music. "Now I think we are into a whole other thing where people are using the music in a freer way."

His latest project mixes klezmer with hip-hop, house and punk.

"There is such a variety today," says Aron Zelkowicz, the festival's founder and director. "It isn't just wedding music anymore, [and] it is not just Jews that listen."

Henry Shapiro, leader of the locally based Steel City Klezmorim, began playing a traditional klezmer instrument, the upright bass. But recently he integrated guitar.

"For the longest time I thought guitar was not a legitimate [klezmer] instrument," he says. But later he began to question that. "There have been many changes in the lead instrument."

Indeed, even as characteristic a klezmer instrument as the clarinet was not a part of the early typical klezmer instrumentation of the 17th and 18th centuries (lead and second violin, cimbalom, bass or cello and flute). It was added in the 19th century.

Bands of professional Jewish musicians, called klezmorim, increased in size and incorporated other lead and accompanying instruments. However, much of this music was not written down or recorded and has been lost. Documentation started in earnest when Jewish immigrants came to America.

"The majority of recordings of early klezmer music were made in the United States, and so that's our precious source of this music," says Krakauer.

But even this archival material cannot be called a definitive documentation of the genre because klezmer is so varied. It has always been in flux, influenced by the people of the regions in which it existed.

"The klezmer music in Eastern Europe was in the shtetls, where there was cross-fertilization with Gypsy music, Russian, Ukrainian music, whatever music of the region people were in," says Krakauer.

Steel City Klezmorim -- from left, Steven Greenman, Tom Roberts, Henry Shapiro and Alex Fedoriouk -- will play at 7:30 p.m. Wednesday at Club Cafe. Click photo for larger image.

Yet that influence pales in comparison to the musical spectrum available today in the current klezmer revival.

"People had so many fewer choices and influences [before] the advent of recordings. That is the legacy musicians from my generation are handed," says Krakauer. "We [have] this huge world influence around us."

Curiously, perhaps the biggest factor in the recent boom of klezmer in the United States was its relative disappearance in the middle of the last century.

"When those recordings were made in the '20s, those were immigrants fresh off the boat, so they were still connected," says Krakauer. "In the late '50s and early '60s, when Jewish Americans really were trying to shed their Jewish identity and assimilate into the general culture, klezmer music started to disappear."

"Within 15 years, most of [Jewish immigrant] music was blended with swing and Latin and various kinds of popular music," adds Alan Bern of Brave Old World.

"When I first heard the music, I didn't know what klezmer was, either," says Shapiro.

Klezmer would bounce back with a revival in the '70s as many American Jews began to explore their cultural identity. Michael Alpert (later of Brave Old World), Andy Statman, the Klezmorim, the Klezmer Conservatory Band, Kapelye and Henry Sapoznik (whose KlezKamps were crucial to klezmer's transmission) were among the prominent figures that re-introduced the music to the greater American public.

"There was this great euphoric feeling that we uncovered this music that had to do with our own backgrounds," says Bern, who co-founded Brave Old World in 1989.

At first, the revival was somewhat mired in attempted authenticity.

"The first bands were tremendous fun live, and put out horrible, stiff albums," writes Ari Davidow on the extensive Web site klezmershack.com. "There was irony in modern folk musicians reviving a folk pastiche by trying to learn songs note for note from old 78s."

"We were dealing with scratchy old recordings because there wasn't by and large any published music," says Bern. "We had to learn this like a language that wasn't being spoken anymore. Most of the musicians in the klezmer revival have spent thousands of hours listening to old scratchy 78 recordings and trying to sift through the sand and dig out the gems."

But soon the scene began to loosen, picking up the klezmer spirit of innovation and adaptation as well as the notes.

"I listened very carefully and did transcriptions, but I also wanted a natural mutation to occur," says Krakauer. "In fact, I was able to develop my own sound and style. There is that sense of trying to connect with the past and at the same time trying to do something new."

Krakauer first looked to jazz, then joined with the Klezmatics (who pioneered a sort of rock and ethnic-tinged klezmer) and later contributed to a string of classical/klezmer crossover successes. The latter will be the focus of Krakauer's appearance on the festival. He will solo in Osvaldo Golijov's "The Dreams and Prayers of Isaac the Blind," Prokofiev's Overture on Hebrew Themes and other works.

Steel City Klezmorim will present a night of traditional klezmer but with the twist of using a guitar as a lead. Brave Old World will present a concert of klezmer music that existed against all odds in the Nazi ghetto of Lodz, Poland.

"The question was how to communicate to audiences what was going on then and make it Brave Old World music," says Bern. "What we wound up doing was creating a program that combined our own music and the music of the Lodz ghetto. It's a song cycle of 120 minutes without a break."

In addition to being a toe-tapping affair, the cycle of the festival's concerts may open some ears as to just how diverse klezmer can be.