Improvisation is at the core of Alvin Singleton’s gorgeous string trio Be Natural (1974), its title a pun on the B-natural note that is passed around throughout the piece but is always droning, a groaning river flowing underneath jittery textures... near the end, a ferocious duet for viola and cello gave way to a passage of music that was quiet, mellow, simply gorgeous.

• Zachary Woolfe, The New York Times Be Natural is a work for any three bowed stringed instruments. It brings into the Singl...