Trapezoid, a West Virginia string quartet, first gained recognition in folk music circles seven years ago playing rags, reels and jigs on four-hammered dulcimers. Since then, the band has made some changes: Its repertoire presently includes more original and contemporary material, and exquisite vocal harmonies have been added to the mix. The combination of guitar, mandolin, violin, cello and bass brings richer texture to the group's music, which continues to make imaginative use of the dulcimer and its musical cousin, the bowed psaltery.

As anyone who was at the Wolf Trap Barns Saturday night can attest, these changes now allow the band to chart an unpredictable and delightful course in concert. The dulcimer's sprightly sonorities played by Paul Reisler were first heard on Si Kahn's "Wild Rose of the Mountain," and the instrument later imparted a dancing pulse to a brisk instrumental inspired by an African kora player.

But the evening wasn't dominated by any one instrument. The band nimbly moved from Irish fiddle tunes and mountain ballads to jazz novelties and some lovely songs written by the group's mandolinist, Lorraine Duisit. In addition to being a fine songwriter, Duisit is an exceptional singer whose voice in phrasing occasionally recalls Joni Mitchell. More than anything else, the band's music has benefited from Duisit's increased participation.