The companions of the title are Dar Williams's songs, which the singer-songwriter revisits here two different ways. Disc one's dozen tracks are back-catalogue tunes re-recorded acoustically with guests; disc two is a generous 20-track retrospective drawn from the original albums. Why pair them? Why not? Williams escaped the limiting acoustic-troubadour trap all the way back on her 1996 sophomore album, often setting her compositions to muscular backings that put her squarely into the burgeoning alt-folk box. By stripping them down to something more spare, she allows the songs to be re-examined more closely, placing the emphasis not only on her always-worthy words but also on the intricate melodies she crafted. The guests — among them Mary Chapin Carpenter, the Jayhawks' Gary Louris, Nickel Creek's Sean and Sara Watkins, and Patty Larkin — serve primarily as harmonizers, adding a layer of vocal richness to tunes that might have been subsumed by the louder instruments in their original incarnation. A few of those recast songs also turn up on the anthology disc, and that allows for comparisons. "Spring Street" loses its ringing guitar and spiffy organ riff; "When I Was a Boy," fairly quiet on the original album, isn't that different at all. But the greater value of the bonus disc is its role as tidy summation of where Dar Williams has been, even if it leaves little clue as to where she might yet go.