Unlike many folk groups, Trapezoid is not an easy band to sing or clap along with -- the harmonies are too unorthodox, the rhythms too precise, the phrasing too personal, the playing too exacting. Though the sold-out crowd at the Wolf Trap Barns Friday night managed to clap and sing to a few tunes by this West Virginia quartet, mostly the audience sat back and marveled at the sophisticated complexity Trapezoid found in the world's rural acoustic musics. They should hammer the final nail into the coffin of the idea that folk music is simple or primitive.

Playing 11 different instruments expertly, the foursome constantly shifted its lineup from the material on their own four albums to band member Lorraine Duisit's solo album and the band's projects with John McCutcheon and Holly Near. But at all times, Trapezoid maintained an essential balance that gave each piece a satisfying completeness. The sustained notes of Freyda Epstein's fiddle and Ralph Gordon's cello were balanced by the complex rhythms of Lorraine Duisit's mandola and Paul Reisler's hammered dulcimer. Epstein's earthly alto balanced Duisit's more ethereal soprano; Reisler's finger-picked guitar balanced Duisit's bowed psaltery.

Though they bounced jauntily through bluegrass and swing dance tunes, they were at their best on traditional and original ballads where the stretch and clasp of harmonies were breathtaking.