There was never more than one instrument -- an acoustic guitar -- onstage Thursday night at the Birchmere for a show that began with Nerissa and Katryna Nields and ended with Dar Williams. While the Nields sisters played up their strong harmonies, Williams performed alone, her voice filling the room with comforting warmth and chasing away the storm clouds outside.

After her first song, Williams admitted that she hadn't prepared a set list, and handed a notepad and pen to a friend sitting up front, deputizing her to come up with a program for the evening. She kept things casual, turning to her fans for help remembering lyrics and interrupting herself in "What Do You Love More Than Love" to exclaim, "This is a little higher than I usually do it, but I like the challenge!"

That upbeat demeanor pervaded her set, as Williams's good-natured sense of humor shone through in both her storytelling and her songs (a post-breakup phone call described in "The Blessings" elicited giggles from the crowd). And although she did pound her guitar on the speedy "Are You Out There," mostly she lingered over her tunes, savoring every moment of "The Beauty of the Rain" and the outstanding "Mercy of the Fallen."

Williams had joined the Nields during their set to perform "I Know What Kind of Love This Is" (a song Williams's trio Cry Cry Cry covered), and the two sisters returned to the stage for Williams's last few songs. The Nields sang during the songs' choruses and danced in place during the verses, their huge grins reinforcing the camaraderie among the performers and the jubilation of the evening's music. -- Catherine P. Lewis