NEW YORK, NY.- “Can your hiccups be even bigger?” composer Natacha Diels asked the JACK Quartet on a recent morning.

“I was thinking there were differences in how you were leaning back on the flamingos,” she added, and, addressing the cellist, said: “Jay, your owls are a little unconvincing. Maybe a little more jowl in your owl?”

Somehow, this bizarre code would translate into meticulously uproarious art. Diels and JACK had come together in an airy room at the Mannes School of Music in Manhattan to rehearse her “Beautiful Trouble,” a five-part piece premiering in February that brings together surreal short films and just-as-absurdist live performance.

Diels calls for the four musicians to hiccup, as well as make clicks, dings, odd little movements, head rolls and maniacal grins, among much else. Flamingos and owls are drawn in the score as notation for a full-body unfurling motion and shudder. At that morning rehearsal, JACK’s usual instruments — violins, viola and cello — were still in their cases at the edge of the room.

A few days before, I had spent time with the venerable Emerson String Quartet as it prepared to give its final concerts, with music of Beethoven and Schubert. Compared with that, this JACK rehearsal didn’t feel like a different group or a different piece; it felt like a different world.

“The performance practice can be kind of far away from the Classical-Romantic continuum,” Jay Campbell, the quartet’s cellist, said with winking understatement in an interview alongside his colleagues a week later. “And we gravitate to that.”

“That” can mean a lot of different things. The group — Campbell, violinists Christopher Otto and Austin Wulliman, and violist John Pickford Richards, all in their mid-30s to mid-40s — can do, with equal aplomb, austerely earthy arrangements of Renaissance and medieval pieces, the eclectic folk jam of Gabriella Smith’s “Carrot Revolution” and Diels’ fanciful choreography.

John Zorn’s ferociously fast thickets of notes and Catherine Lamb’s glacially shifting microtonal drones are both JACK specialties; on Friday, the group will perform Lamb’s 90-minute magnum opus “divisio spiralis” at Yale.

With that sprawling stylistic range and its technical mastery, its curiosity about eminent and student composers alike, its precision and passion, JACK has, since its founding in 2005, become one of contemporary music’s indispensable ensembles.

“There’s almost nothing they can’t do,” said composer Amy Williams, who is at work on a new JACK commission. “So in a way, it’s like writing for electronics, with no human limitations. That can be exciting, and also terrifying.”

Soprano Barbara Hannigan, who has toured with the group, said: “It’s a very disciplined yet manic virtuosity. And somehow they’re also very calm at the center of all that virtuosity. They are super, super centered. I’ve worked with quartets with a specialty in modern repertoire, but there’s nobody like JACK.”

The group formed in the heady atmosphere for new music at the Eastman School of Music in Rochester, New York, in the early 2000s. The players in the original lineup — Richards, Otto, violinist Ari Streisfeld and cellist Kevin McFarland — were united by decisive encounters with the work of German composer Helmut Lachenmann, a master of sonic extremes. Lachenmann traveled to Toronto to coach three of the JACK members in his first quartet, “Gran Torso,” and the group flew to a festival in Mexico with other Eastman musicians to continue working with him.

“I am their father, or something — their grandfather,” Lachenmann, who turns 88 this month, said with a laugh recently. “They were totally precise, and very musical. And there is for me one word that is very important: They are serene. When I met them, immediately it was clear, the honesty and the concentration. I don’t find better groups for my music than them.”

Otto recalled, of their early work on Lachenmann’s third quartet, “Grido”: “We could sense that it was just the tip of the iceberg. Just the depth of this music — I’d never encountered something like that before, the thought that we could just continue practicing this piece for a really long time.”

They chose to call the group JACK, an acronym of their first names — at first a jokey nod to Lachenmann, whose “Grido” is named after the members of the Arditti Quartet. But the players also liked its slightly ironic all-American quality and its modesty.

In the first years, the group played only sporadic concerts, and they weren’t usually glamorous. JACK often performed at the Tank, then on Church Street in lower Manhattan, where cockroaches would sometimes scamper over the musicians’ feet while they played.

Lachenmann put in a good word with WDR, the influential radio network in Cologne, Germany, which invited JACK to play and record all four of Iannis Xenakis’ quartets, a feat not yet attempted. Released in 2009 as part of Mode Records’ complete Xenakis project, it made the group’s reputation.

“We got paid to record it, which is crazy,” Richards said. “And that album introduced JACK to a lot of people.”

It established the quartet as youthful masters of daunting modernism, as did a live recording of a 2011 performance of works by Xenakis (“Tetras,” an intense calling card), Ligeti, Cage and Matthias Pintscher at Wigmore Hall in London. But the flood of repertory and touring soon grew trying.

“It’s hard to do more than 70 or 80 concerts a year with all new pieces,” Richards said. McFarland wanted to move to Colorado, where his partner lived, and Streisfeld wanted to stop traveling so much and take a steadier teaching position.

They left the group in 2016, and while Otto and Richards were committed to keeping JACK going, it was, Richards recalled, “surprisingly hard to discover people who wanted to throw their whole lives into it.” But Campbell and Wulliman, both well regarded in the cozy contemporary-music community, fit the bill.

“I had been playing in professional new-music ensembles in Chicago,” Wulliman recalled. “But to sit down with these guys to read through ‘Tetras’ — whoa, I have never, ever, ever experienced anything like that. Being able to just get through something that easily. The ease of the music moving forward.”

JACK is not one of those businesslike quartets that travels separately and meets up just for soundchecks and performances. “I still like spending time with them,” Wulliman said. They go on hikes and search out new restaurants together on tour — and, when road trips are involved, always sit in the same configuration in the car, with Richards at the wheel.

The four have mock fights about things like whether they should play Ralph Shapey’s astringent music. (“I’m dying to do Shapey,” Otto said; “I’d rather die,” said Wulliman.) But when they’re rehearsing, they speak in genial fragments, completing one another’s sentences and doing much more playing than debating.

Going through an arrangement of a piece by 16th-century composer Nicola Vicentino, Otto, who was doing a harsh, very contemporary-sounding bow stroke, asked, “Does it feel over the top with the sweeping stuff?”

Wulliman thought a second, and answered, “I’m not ready to pass judgment yet.” And they moved on.

In the interview, Otto said: “We’ve always gotten along, but there’s been an evolution in how we communicate — with our playing as well as our words. Sometimes, early on, we would get overly conceptual or just talkative at rehearsals, and it wasn’t really that productive.”

Since those beginning years, using the Kronos Quartet as a model, JACK has been organized as a nonprofit — Lachenmann co-signed the articles of incorporation — to allow it to raise money, commission pieces and start initiatives on its own, rather than waiting for partner institutions.

“It feels like institutions are just a little behind,” Campbell said. “I want to be more in front of it.”

In 2018, the group became the quartet in residence at Mannes, a milestone for its artistic and financial stability. Its budget in 2010 was $120,000; it’s now $700,000, separate from the members’ Mannes salaries — and large enough for JACK to have hired a full-time executive director, Julia Bumke, in 2020.

As its 20th anniversary approaches, the group is focusing on expanding its fundraising to include more individual givers amid the grants and foundation support, as well as fortifying its already robust commissioning activities, including the JACK Studio program, which offers funding for new scores as well as a range of mentorship and performance opportunities.

When the quartet believes in a composer, it truly commits. “As we worked together,” Lamb said, “something clicked: ‘I can really write what I want to write for these people. I don’t have to hold back. I can explore what I want to explore.’ So I let myself go.”

The result was “divisio spiralis,” an epic, 13-part experiment in delicate yet rending, mesmerizing harmonic changes that demands hyper-exact intonation to make its impact.

“The last time I heard them play it live,” Lamb said, “I was overwhelmed by how much it had grown since the premiere in 2019. They had more clarity in reaching the sound colors together, finding the right kinds of balances. It’s more and more seamless, more and more musical.”

That commitment to finding the music — the sheer beauty — in what could be merely exercises in complexity, to treating every composer like a distinct style that can be ever more fully inhabited, is what sets JACK apart.

The group said it was a little intimidated by the difficulty of Amy Williams’ music. But Williams, whose new JACK piece relies heavily on hocketing, the medieval technique of alternating rhythms so lines interlock like a zipper, said that was unlikely.

“They have absorbed so much music — working with students, premiering pieces, large-scale composer projects,” she said. “It’s quite extraordinary how much they’re processing. Challenging them is no longer on the table.”

This article originally appeared in The New York Times.