After performing two sold-out duo concerts at downtown Manhattan's Bowery Ballroom in December 2015, pianist Brad Mehldau and mandolinist Chris Thile decamped to Avatar Studios in Hell's Kitchen, where they spent three more creatively charged days recording together. They brought their show into the studio, exploring original compositions and an impressively eclectic stack of covers. Their approach as a duo was as unorthodox in concept as it proved to be thrilling in execution, employing just piano and mandolin, along with Thile's elastic and expressive voice (and, on "Scarlet Town," Mehldau's vocals too). The sessions yielded a brilliant, often freewheeling, live-in-the-studio double album, Chris Thile & Brad Mehldau, the first joint recorded effort from these two disparate yet equally formidable talents. As New York Times jazz critic Nate Chinen put it after a 2013 concert, "Chris Thile and Brad Mehldau come from different worlds but the same species, and whatever feels unlikely about their pairing is eclipsed by what feels perfectly natural. Mr. Thile, the mandolinist and singer with Punch Brothers, is a progressive-bluegrass pacesetter; Mr. Mehldau is the most influential jazz pianist of the last 20 years."

"I think we've found a way to play with each other that is quite unique," says Mehldau. "It highlights the string-instrument aspect of both instruments. Chris and I are both influenced by guitar-oriented music, so I think we wind up 'playing guitar' for each other a lot, only I'm doing it on piano and he's doing it on mandolin. Chris also plays drums a lot for me—you can hear that on a track like 'Scarlet Town,' where he gives me a kind of back beat. Even though that's a very small instrument he plays, there's a large world of musical sound that he conveys. So probably a lot of what's special about this project for me is the way we get beyond our instruments."

"I don't know anyone with bigger ears," Thile says. "He instantly processed the mandolin's limitations. It's precise but very delicate instrument, whereas the piano is like a superhero. I was a little nervous about the disparity until we actually started playing together. He's wonderfully sensitive to how easily overpowered the mandolin can be, but also manages to completely avoid that sort of 'walking on eggshells' sound. He can play higher and lower than me, much lower and a little bit higher, which he's mindful of as well, always listening, sometimes clearing out a spot for me, sometimes propping up an idea by meeting me in that spot. And then there's the nice rhythmic connection ... Both instruments are about as percussive as chordal instruments can be, and Brad is such a deep pocket. I think that was of great interest to both of us: setting up these open fields of rhythm to romp around in."

This two-disc set features expansive takes on several covers, including a heartbreakingly melancholic rendition of Joni Mitchell's "Marcie," a version of Elliot Smith's "Independence Day" that forsakes lyrics to spotlight its evocative melody, and a rollicking, practically ribald version of Bob Dylan's "Don't Think Twice, It's All Right" that the duo often uses as its closing number in concert. Mehldau's arrangement of the standard "I Cover the Waterfront" is a daringly slow burn, the melody emerging like an outline in the fog, and Thile matches it with a smoldering performance of his own. Two of Thile's originals address his relatively new role as a father with humor and honesty. The set opens with the bold stroke of the co-written "The Old Shade Tree," which features what must rank as one of Thile's most unfettered vocal performances. It doesn't take him long, with the encouragement of Mehldau, to really let go.

"Playing comes more naturally to me," Thile admits, "I'm comfortable just letting it rip, whereas I'll sweat singing in the studio until I get it 'just so.' But I didn't really feel the need to do that during this session, to be so careful with the vocals. In addition to everything else Brad's an extraordinary accompanist, unbelievably responsive, supportive, and encouraging. You might hear me tentatively go for something on this record and Brad's response to it on the piano is 'Yeah man, do that.' I remember the physical feeling of those 'Old Shade Tree' takes: Brad would do something badass and I'd suddenly feel ten feet tall and bullet proof, like I could just rear back and go for something knowing everything would be okay. He would catch me."

"I've rarely been with a singer who is as relaxed in the studio," counters Mehldau. "Chris clearly knew when he wanted to get something better, and we would just do another take, which was fun. It wasn't one of those exacting, piece-by-piece sessions; it was just lots of fun music-making. There are no vocal fixes on the final product in terms of Chris doing something later alone. We did everything together in the same room. There were only a handful of edits where we grabbed part of another take that was better—and it wasn't necessarily the vocal we wanted to fix; it might have been something else. The takes that we mixed are mostly live, and to me that's amazing, but it's also just the way Chris worked in this project. Everything was just there already. In that sense, it felt like a lot of the jazz records I've made."

Thile had long been a Mehldau fan. He recalls that fellow Punch Brother Gabe Witcher—who lent his editing expertise to these sessions—introduced him to Mehldau's Art of the Trio 4: Back at the Vanguard, a 1999 live set that, admits Thile, "may be, along with Kind of Blue, the quote-unquote jazz record I've listened to the most. There was a purity of spontaneous development that I hadn't really encountered before. I honestly don't think there's ever been a musician with a clearer path from their mind, heart, and soul to their fingertips than Brad."

Mehldau's name on the Nonesuch roster was a big draw for Thile when he was invited to join the label. Nonesuch's Bob Hurwitz saw their similarities, as he remarks in the liner notes he penned for this set: "I am always amazed at how broad their interests are, how accepting they are of all kinds of music, and how much they have embraced the music of our time as well as the music of the past." And, with no motive in mind other than to share his enthusiasm for Thile and his band-mates, Hurwitz invited Mehldau to join him at the Bowery Ballroom for one of the earliest Punch Brothers gigs.

"It was electrifying," recalls Mehldau, "and I was instantly a fan, right there. I heard immediately that Chris wore a few different hats at the same time but was strong at everything. It's like he is three or four musicians at once, and each one would kick ass in its own right: A bandleader, a singer-songwriter, a virtuoso instrumentalist, and an inspired improviser. All of those different musicians within the one guy that is Chris appealed to me equally!"

Mehldau and Thile both performed at a Wordless Music event in October 2008 that served as a Barack Obama campaign benefit. They played separately but were clearly on the same wavelength: This show marked Mehldau's first solo recital of composers from the classical canon, including Bach, and Thile too would perform pieces by Bach as well as original compositions. In March 2011, as part of his Richard and Barbara Debs Composer's Chair residency at Carnegie Hall, Mehldau invited Thile to share a bill with composer Gabriel Kahane, and he sat in for part of the set. As Thile recounts, "We did a couple of things together that night and had so much fun. Walking back to the dressing room afterwards we were both saying, 'Man, we really should do this from time to time!' I think part of it was the immediate wealth of shared influences. That thrill of saying, for example, 'Hey, do you know that Fiona Apple song…' and the other going 'ooh, hell yeah, kick it off!' A lot of these things are borne of us kind of misremembering how some of these songs we love go until the song has changed somehow, chemically, fundamentally—and then maybe there's a reason for us to play it rather than just listen to the record again."

Later in 2011, Thile joined Mehldau at Wigmore Hall in London, where Mehldau was curating the venue's inaugural jazz series, for their first official performance playing duo. A music critic for the Guardian wrote that "their musicality and sympathy for each other's emerging ideas made it an unexpected tour de force." When the pair embarked on a short American tour in 2013, the Boston Globe noted, "The two proved admirable partners. They took turns soloing or comping for each other, Mehldau often providing grounding left-hand rhythms, Thile laying down percussive riffs with dampered, toneless chording. But the high points came when they took off in contrapuntal flights, then fell into cadences of dramatic closure." And the Washington Post stated, "Their complex work translated to plain-faced beauty: simple, direct and exquisite."

Chris Thile enjoyed success at an early age with the Grammy Award–winning roots music trio Nickel Creek. But his musical and intellectual curiosity has taken him far afield, from Punch Brothers to collaborations with classical cellist Yo-Yo-Ma, bassist Edgar Meyer, and guitarist Michael Daves, among others, and, in 2012, garnered him a MacArthur Foundation fellowship (a.k.a. "Genius Grant"). Thile continues to take the mandolin into uncharted territory: For a 2013 album produced by Meyer, he performed Bach sonatas and partitas for solo violin. Now, as the new host of the venerable public radio show A Prairie Home Companion, he displays both his tremendous knowledge of and boundless enthusiasm for the widest range of music and comedy.

Mehldau's tastes are just as deep, as his recent eight LP/four-CD compilation, 10 Year Solo Live, featuring interpretations of work from Brahms to the Beatles, John Coltrane to Nirvana, attests; the New York Times calls it "an ambitious self-portrait." Mehldau and saxophonist Joshua Redman just released their own live album of duo performances, the Grammy–nominated Nearness, and Mehldau also received a nomination for his latest Trio disc, Blues and Ballads. His collaborations have included the Love Sublime song cycle with soprano Renée Fleming, duet and quartet recordings with guitarist Pat Metheny, two albums with Jon Brion, including his best-selling Largo; and a recent, rhythmically experimental pairing with drummer Mark Guiliana, Mehliana: Taming the Dragon.

Chris Thile & Brad Mehldau captures a moment, an open-ended one, drawing us into the middle of an ongoing creative conversation between two virtuosic artists, label-mates, mutual admirers, and friends.

—Michael Hill