I’m thoroughly enjoying a set by the Chelsea-based bluegrass trio Thunderwüde when I discover that the trio is actually supposed to be a quartet. Mandolin player and vocalist Jason Dennie is out of town, leaving guitarist George Merkel, bassist Tommy Reifel, and fiddler Wesley Fritzemeier to hold down the fort, with Fritzemeier taking over mandolin duty for the evening.

A quick look at some past YouTube videos of the band reveals that Dennie usually acts as frontman for the group, handling lead vocals and kidding around with the audience. And yet without him the group still acquits itself exceedingly well, playing bright, jammy covers and originals and aptly engaging a crowd of approving fans at the Chelsea Alehouse. Even out of their usual element, Thunderwüde is a group of truly versatile pros.

The trio I’m seeing tonight is also three-fifths of the Ben Daniels Band, the folky outfit often seen opening for bandleader Ben Daniels’ father, Chelsea native son Jeff Daniels (see August 1-2 events listings). And their musical talents aren’t limited to Americana; Fritzemeier also leads a jazz group that plays weekly at the Alehouse. Thunderwüde itself got its start in a series of informal sessions at the Alehouse. It eventually picked up a name and a faithful following, sometimes filling the Alehouse to capacity in its weekly engagement.

It’s not hard to see why. The laid-back way these musicians gel as a group is fascinating to watch. One set begins with Fritzemeier alone onstage, gazing placidly into the middle distance as he jams quietly on his mandolin. It seems almost as though he’s just idly warming up for the next round, but he’s shortly joined by Merkel and then Reifel, who jump in with his melody and turn it into a full-length set-opening bluegrass jam. The band’s set this evening is light on Dennie’s warmly humorous original tunes, but the selection of covers is broad, often crowd-pleasing, and indicative of a thorough knowledge of musical history. The group delivers a swinging cover of Meghan Trainor’s 2014 hit “All About That Bass” but also digs deeper with a rendition of the early John Prine tune “Paradise.”

The group recalls an old-school musical tradition as they stand around a single vocal mike, harmonizing closely, with Fritzemeier and Merkel trading off on lead vocals. Merkel and Reifel provide a solid rhythm section on guitar and upright bass, while Fritzemeier takes most of the instrumental spotlight. Although it’s not his usual instrument, Fritzemeier is an exceptional talent on the mandolin, executing dizzyingly complex solos with precision. The band also displays an easy repartee with its audience. “This next song is in the key of B, which means it’s really hard to play unless you have a capo and your name is George,” Fritzemeier cracks at one point.

They’re one man down, but none the weaker. In one night the group thoroughly and unassumingly proves its adaptability, and the crowd demands an encore at the end. The band hardly hesitates for a moment. Thunderwüde retakes the stage and just keeps jamming.

Thunderwüde–the quartet–plays the Ark August 27.