, marking the occasion with a show at New York's Beacon Theatre. The two last came together in 1970, when John opened up for Russell at New York's Fillmore East. "His music takes me back to a wonderful time in my life, of music and experimenting, people sharing what we had," John said to Rolling Stone about Russell, who had hits with "A Song For You" and "Delta Lady" in 1970. "It pisses me off that he was forgotten about."

Russell, John, producer T Bone Burnett and Elton's longtime lyricist Bernie Taupin all had a hand in writing the album's 14 tracks, which feature Elton and Russell playing piano simultaneously. ("No one uses two pianos on a record anymore, since Phil Spector, probably," John said.) Russell was sitting in his Nashville home one day in 2009 when Elton phoned about working together. The recording sessions happened in Los Angeles earlier this year, just after Russell underwent brain surgery. "Leon had the surgery three weeks before we went into the studio — he was probably still under anesthetics," John said. "But you could see the music starting to wire him back up. By the second week, he was killin' it on piano. The more he did, the more he came to life." Brian Wilson, Neil Young, Robert Randolph and Booker T. Jones also make guest appearances on the album. The first single, "If It Wasn't for Bad," a dark duet written by Russell, is available on digital music services now.

For more on The Union and our interview with Russell and Elton, check out our In the Studio piece with the legendary pair in our All Access section.