Many musicians, perhaps especially ones with early immersion in traditional folk music, eventually decide if they will continue playing that music just as they received it or expand the boundaries of their tradition. Few have blended these options more gracefully than fiddler Alasdair Fraser.

Decades into a highly acclaimed career, Fraser still plays the centuries-old traditional dance tunes and airs of Scotland, and he’s already been inducted into the Scottish Traditional Music Hall of Fame. But in his playing and composing, he has also ventured far beyond the borders of that music, incorporating modern fiddle techniques, improvisation, and, notably, the cello.

Young American cellist Natalie Haas has been touring and recording with Fraser since 2003. While her Celtic creds are impeccable (she’s been studying and playing that music since she was a young child), she’s also Juilliard-trained. She’s likely as capable on Schubert sonatas as she is on Scottish strathspeys, and she employs a myriad of classical and modern cello techniques, from the bass-like plucked pizzicatos to percussive, damped chopping and guitar-like strumming. This allows their duo, the “wee fiddle” and the “fiddle grande,” a remarkable range of rhythmic, melodic, and harmonic possibilities. Each can take the melody or supply a parallel harmony, a backing rhythm, or repeating ostinato or play long held lines against the tune. They can sound like a classical chamber ensemble, a rollicking dance band, or a drummer-less Hot Club swing jazz band–sometimes all in the same tune. When they were at the Ark five years ago, Fraser quipped, “This is our band; it’s not the case that the other half of the string quartet didn’t show up.” But if you had closed your eyes, you might have thought they had.

Cellos are not commonly associated with traditional Scottish music, but they haven’t always been strangers. “They were taken into captivity by Western orchestras,” jokes Fraser, where they became serious and scholarly. “The cello in the eighteenth and nineteenth centuries used to comprise the rhythm section in Scottish dance bands.” Fraser’s off-the-cuff observations, delivered in his droll Scottish burr, are another highlight of their shows.

Fraser and Haas return to the Ark on March 14 with a new recording. Ports of Call has their brilliant versions of some old Scottish tunes, striking originals, and tunes from Scandinavia, France, and Spain. On their last visit to the Ark, they signaled this new direction when they opened their second set with the loveliest version I’ve ever heard of Swedish guitarist Roger Tallroth’s Josefin’s Waltz.