The duo of guitarist Tuck Andress and vocalist Patti Cathcart perform with in-ear monitors. “The result of these is that everything but Patti and me disappears while we are playing,” says Tuck. They draw the listener into that closed circle, in which the energy is quiet but intense.

It’s remarkable that they can sustain that intensity; Tuck and Patti got together in 1978, supposedly knowing within seconds of meeting that they had found a long-lasting musical partnership, one that soon turned into matrimony as well. To those of us ordinary mortals whose long-term relationships require a delicate balance between togetherness and distance, these full-time pairings seem nothing short of miraculous, but after thirty years the music of Tuck & Patti continues to deepen into a profound fusion of souls.

Tuck & Patti can cause trouble for the people who put music into categories, but jazz is the best word for what they do, even though the combination of vocalist and guitarist is a rare one in the genre. Tuck is capable of snap decisions and intricate details devised on the spur of the moment, and he can shift easily from steady rhythmic accompaniment in a contemporary pop song to a really dizzying variety of attack in music by the likes of Gershwin. Patti has an alto voice something like that of Detroit’s Anita Baker, but quieter: it can either sail into a light upper register or broaden out into a rounded tone at important moments. The music on recordings never goes above conversation level, putting Tuck and Patti among the few American performers who occupy the lowest 10 percent of the dynamic spectrum.

The latest Tuck & Patti album, I Remember You, is devoted to Gershwin tunes and other classics of American song. They also perform newer standards like “I Will” and “Time after Time,” and they have the conviction and originality to escape the slightest hint of triteness even in very familiar material. Occasional humorous songs, solo numbers by both Tuck and Patti, and offbeat experiments like an acoustic version of Jimi Hendrix’s “Castles Made of Sand” provide variety. But the common denominator of most of the duo’s songs is love, naturally enough, and if romance on a midwinter night is on your agenda, their concert will be a profitable stop. Tuck & Patti also perform quite a few of Patti’s originals, lending the music a folkish flavor. “Patti writes and arranges; I am just the orchestra,” Tuck says. He has the talent to deliver on that little boast.

Tuck & Patti perform on Tuesday, December 16, at the Ark, a venue whose increasingly diverse range of bookings is setting it apart from other clubs around the country devoted to folk and acoustic music. This club is one of the few places around that can offer the deep, quiet focus this remarkable music deserves.