It’s a running gag at performances of the Wainwright-McGarrigle clan, recently called the “dysfunctional first family of folk-pop.” Patriarch Loudon Wainwright III will introduce his longtime collaborator Chaim Tannenbaum as “the Jew who is going to sing a gospel song.” At a family Christmas concert, son Rufus Wainwright announces “an exciting song from our friend Chaim who’s going to do ‘Blue Christmas.’ It’s a joke!” Matriarch Kate McGarrigle featured Tannenbaum singing the impassioned line “I’m bound for Calvary” on the song “Dig My Grave.”Tannenbaum is happy to play along: “From the house in which I grew up, we could walk to three kosher bakeries, three synagogues, four Jewish day schools,” he’s said before launching into “Blue Christmas.” “What I’m trying to tell you is that Christmas didn’t really mean a great deal to us. But we did have Elvis Presley’s Christmas records.”The story of folk legends Loudon Wainwright, his ex-wife Kate McGarrigle (who died in January 2010) of the McGarrigle Sisters, and their children Rufus and Martha Wainwright has always revolved around extended family, and Tannenbaum has been a constant presence in their life and work. His credits on their albums and live shows include songwriting, lead and backing vocals, banjo, harmonica, guitar, mandolin, recorder, and saxophone; he was also a co-producer of Therapy, arguably Loudon’s best album. According to Loudon, Tannenbaum is “all over” his box set, 40 Odd Years, which was released on May 3. Tannenbaum’s influence is no less pronounced on the McGarrigle Sisters’ three-disc compellation Tell My Sister, which was released on the same day. And, together with Emmylou Harris, Nora Jones, and others, he’s to be a key player in Rufus and Martha’s musical tribute to their mother at New York’s Town Hall tonight and tomorrow.Despite Tannenbaum’s ubiquity in the 40-plus year saga of the lives and music of Loudon, Kate, and their children, there is very little information available on him, other than a few grainy concert clips on YouTube and a Facebook page put together by adoring former students from Dawson College in Montreal, where he taught philosophy for many years before his recent retirement. Asked if there is any relationship between his life as a philosophy professor and a musician, he responds that there is no link whatsoever: “I may as well be two unrelated people,” he says. Outside his work with the family, Tannenbaum’s musical legacy is virtually nonexistent.He is painfully reluctant to give himself props for any of the family’s accomplishments. Asked about his collaborations with Loudon, Tannenbaum seizes on a hockey metaphor: “The Montreal Canadiens had a line consisting of Hall-of-Fame centre Jean Beliveau, Hall-of-Fame right winger Bernard Geoffrion, and perfectly ordinary left-winger Bert Olmstead. Lucky Bert Olmstead, he had the easiest job in hockey. Loudon had me and David Mansfield play with him. In that trio, I think of myself as Bert Olmstead.”But his “Beliveau” and “Geoffrion” beg to differ. Loudon calls Tannenbaum “my unsung hero.” And Mansfield—a veteran of Bob Dylan’s Rolling Thunder Revue and a founding member of Bruce Hornsby and the Range—is even more emphatic: “Anyone who doesn’t consider Chaim a living legend hasn’t been around folk music long enough,” he says. “Somehow I can’t even imagine Loudon’s career without Chaim’s high keening tenor a third or a sixth above. I can’t think of a single musician who has contributed more to [Loudon’s] work.” Mansfield goes on to explain that “to think of Chaim as anything less than family with Kate and Anna would be a mistake.”The story of how Tannenbaum connected with Loudon and Kate is as entertaining as the collaborations that followed. In the 1960s, Tannenbaum and Kate met as high-school students in Montreal and became members of a local folk band called the Mountain City Four. Tannenbaum eventually moved to the U.K. for graduate school, and Kate moved to New York, where she met and married Loudon. “In 1970, I got a telegram that read ‘Arriving from Copenhagen, Tuesday, Kate,’ ” says Tannenbaum. “When the doorbell rang on Tuesday, it was, unmistakably, Kate McGarrigle behind it, bags in hand, with a tale of marital distress and abandonment to go with. … A week, maybe two weeks, passed in a state of tearful domestic tranquility. Then Loudon showed up at the door saying, ‘I’m looking for my wife.’ Thus, improbably, the friendship was launched.” According to Loudon, they soon began busking on the Portobello Road.Since that fateful meeting, Tannenbaum’s musical friendship with the family has become the bridge for many apparent incongruities. In his different identities, he’s a Canadian Jew known for playing southern gospel and Christmas songs; he’s a bluegrass banjo player named Chaim (as opposed to “Slim” or “Tugboat”); he’s an obscure philosophy professor who has quietly become a folk legend; he’s nearly invisible to the public yet deeply revered by the famous people who work with him.Even his songwriting achievements seem accidental. His composition “Time on My Hands,” which he sings on the album The McGarrigle Hour, would never have surfaced had Kate not found it “on a cassette Loudon was carrying around.” And Joe Boyd, producer of the McGarrigles’ Tell My Sister compilation, recently told Tannenbaum: “I bet you haven’t heard this song ‘Annie.’ I’d never heard it. It’s great.” Tannenbaum responded that he wrote the song.For Tannenbaum, all the contradictions of his life seem smooth sailing. He talks about singing a gospel song in Nashville last year at a concert supporting the release of the Grammy-winning album High Wide & Handsome, Loudon’s record about bluegrass pioneer Charlie Poole. Tannenbaum says that after the show, a local woman of about 75 years old came up to him and said, “You know I never expected in my lifetime I would hear a Jew sing the Gospel like you did.” Tannenbaum explains that there wasn’t a tincture of anti-Semitism in her declaration. It was a matter of wonder: How could she have expected that the simple religious music of her isolated Tennessee childhood would be lovingly sung by a Jew of all people, from another part of the world? “I wonder, too,” he says.Music is the portal through which Tannenbaum has traveled across time, space, and different identities: “I loved it when Chaim and Kate were paired on harmonica and accordion during my French songs, ‘Complainte pour Ste. Catherine’ and ‘Mais quand tu danses,’ ” says Anna McGarrigle, “as if we’d momentarily touched down in some bar in rural Quebec, or when he’d lean in on mandolin along with the guitars, we all got airborne again. When he led us on ‘Dig My Grave’ we became fearless in the face of death.”“In music,” she continues, “it’s important to be transported.”Harold Heft has taught literature and film at the University of Western Ontario and the Hebrew University of Jerusalem. His last article for Tablet Magazine was about Citizen Kane.