Chamber music, or “the music of friends,” was initially written to be performed in a small room—hence “chamber.” These days, music for small groups is performed everywhere from art galleries to multi-story carparks, on street corners by busking string quartets, and onboard helicopters by intrepid interpreters of Stockhausen. At the heart of the chamber-music repertoire is the string quartet (two violins, viola, and cello), a form raised to glorious heights by Haydn, Mozart, Beethoven, Schubert, Bartók, and Shostakovich, for whom it became one of their most intense forms of expression. But the world of chamber music overflows with duos, trios, quintets, sextets, and numerous other combinations of instruments.