Guitarist Mary Halvorson and violist Jessica Pavone are in the thick of New York’s bustling new music scene, routinely erasing the lines that separate free improvisation, jazz, experimental, and pop musics. They regularly play with Anthony Braxton and trumpeter Taylor Ho Bynum, lead their own projects, and participate in countless ad hoc assemblages, but they make some of their most satisfying and personal work as a duo. They’re about to release their second album, On and Off (on the New York artist-run imprint Skirl), and it sounds like they’ve followed their guts even more than they did on their superb debut, Prairies (Lucky Kitchen, 2005).

The pieces twist and glide through all kinds of terrain, both soothing and jarring: jagged counterpoint, smooth unison lines, extended harmony, dynamics that dissolve notions of foreground and background, composition and improvisation. Attempting to classify the songs is an exercise in frustration because they rarely stay in one place, and the pair’s easy rapport allows them to access all kinds of approaches without a fumble. It’s not unusual for a piece to open with the austerity of contemporary classical music only to fold into a plaintive folk song and then into a concentrated blast of dissonant texture. Halvorson is an especially nimble player, with an impressive mastery of the splintery tangles that distinguished Derek Bailey, who finds the narrow pathway that connects confusion and fluidity. On their debut they added some wordless vocals to one track, but here several tunes include genuine lyrics, which they sing together with earnest simplicity, tracing unadorned melodies without a lick of self-consciousness.

The duo performs tomorrow night at the Hideout .