Nickel Creek offers the best of both worlds on 2002’s This Side — its songs display a love of bluegrass tradition, but also a desire to push against the genre’s boundaries. The trio’s sophomore album manages to be serious and quirky at once, reaffirming the band’s technical excellence while acting upon some offbeat impulses. Chris Thile and siblings Sean and Sara Watkins wrap delicate acoustic filigree around slippery rhythms and unexpected chords, binding their sound together with an unforced chemistry. “Should’ve Known Better” combines a skanking groove with a disturbing string arrangement, setting off the nervous sweetness in Sara’s lead vocal. The jittery pulse and jazzy accents of “Beauty and the Mess” and the trio’s cover of Pavement’s “Spit on a Stranger” are both challenging and fun to hear. Nickel Creek’s lyrics are on the moody side and sometimes fall into obscurity — “Brand New Sidewalk” and “Green and Gray” don’t quite make their scenarios clear — but that being said, This Side succeeds on its own terms, transgressing bluegrass conventions and galloping into fresh and exciting territory.