The xx are a band that, live and on record, captivate your attention with the fewest moving parts possible: spare but soft-knocking beats, faint keyboard echoes, guitar and bass sounding single notes at a time—no chords, only negative-space melodies. Over these deceptively skeletal songs—lush R&B by way of austere post-punk—Romy Madley Croft and Oliver Sim trade hushed, intimate vocals. Onstage, they communicate in minute gestures: Jamie Smith busily tapping on his sampler, Croft and Sim swaying imperceptibly. They don't move much, but every move they make is riveting. (Showbox Sodo, 1700 First Ave S, 8 pm, $20, all ages.)