For close to two years, an eclectic cast of classically trained musicians hailing from Milwaukee, Madison, Chicago, and as far away as New York has convened on stage the first Sunday of each month to collaborate. The catch is, none of these musicians—each with thousands of hours of individual experience—have ever played together before, and they may never again. As Unrehearsed MKE nears its 20th lap, the experiment in improvisation has convinced dozens of randomly-paired musicians to set aside mapped out material for one night, and simply see where the music takes them.

The monthly Milwaukee series actually takes root in Baltimore, where jazz musicians Adam Hopkins and Matt Frazao started “Out Of Your Head”—a series that pulled collaborators together to form a makeshift band for one night only, and tasked them with performing a spontaneous set. When the fortnightly event made its way to Brooklyn, it appealed to frequent participant, drummer Devin Drobka, so much so that when he moved back to Milwaukee in 2012, he brought a version of the series home with him. He called it “Unrehearsed MKE.”

“Upon moving back here, I was like, ‘Man, nothing like that is going on here,'” Drobka says. “I wanted to see if something like that would go over well and bring together more of the music scene.’”

Starting in late 2012, Unrehearsed got its start at Sugar Maple. Early editions featured two or three bands that were populated by approximately 12 total musicians, many of whom Drobka knew or had collaborated with in the past.

“I just wanted to throw people together that I’ve never seen play together,” Drobka says. “Sometimes if I see somebody perform out and I like what they do, I want to get them in to see what they can do in this situation. A lot of [band assembly] is just strengths and weaknesses, or just wanting to see what would happen.”

Last summer, the founder of the Milwaukee offshoot called upon two more musicians, adoptahighway’s Barry Paul Clark (with whom Drobka currently collaborates in an improvisation-based project called Argopelter) and Steve Gallam to assist him in the role of co-curator. With three pools from which it could draw, Unrehearsed has been able to choose from about 80 aspiring improvisors from a heft of genres. Past installments have featured Jim Schoenecker (ex Volcano Choir/Collections Of Colonies Of Bees), rapper WC Tank, the ambient electronic stylings of He Can Jog, instrument inventor/musician Hal Rammel, experimental electronic artist Paul Demix, immediate vocal modulation, live painting, and an abundance of musicians with a background in jazz. Though Unrehearsed’s makeup has changed considerably since its modest origin, the focus remains on the pillars of presence in the moment and listening to other musicians. To urge an uncertain outcome, sometimes the curators will withhold band selections until the night of the show, and Drobka admits to occasionally putting musicians of different genres and with drastically divergent stage presence together to see what the mismatched musical pairing yields.

“Sometimes when people know who they’re playing with, they start to make up predispositions,” Clark says. “They think about what [their collaborators] want, which is exactly what we don’t want to happen. We want everybody to just improvise and use the vocabulary of their musicianship to keep communicating.”

Over the first 19 months, Drobka and Clark have witnessed incredible moments in time and the rare “forced” disaster. They consider both outcomes to be valuable sources of growth to participants. Sometimes randomly adjoined musicians will decide to keep working together after the show is over. Clark is currently part of Tontine Ensemble, which formed as a direct result of an Unrehearsed collaboration.

“You kind of find that you need to be really flexible,” Drobka says. “The best musicians throughout time have always been the ones that have been flexible. I think the experience of improvising between different genres, it can change you to be more aware of what you’re playing.”

Unrehearsed relocated to The Jazz Estate in March, and it has been a monthly component of the east side establishment the curators call “a Milwaukee staple” ever since. As intentionally loose and unscripted as the series is, its organizers hope musicians will continue to surrender a few hours of the first Sunday in each month to the unfamiliar.

“What we really want from this is a sense of longevity. This is just a community meeting every month and everybody involved can help continue it,” Clark says.

Unrehearsed MKE #20 will take place Sunday, September 7 at The Jazz Estate. The show begins at 7:30 p.m. and costs $2.