Late string quartets by Mozart and Mendelssohn frame a new work by Joseph Phibbs, co-commissioned by Carnegie Hall. Mozart’s penultimate quartet is noteworthy for its refinement and free-flowing melodies. Mendelssohn’s final quartet intensely churns in its first two movements, reflecting his grief over the death of his sister Fanny. Its slow movement is a beautifully tender song that may well be an elegy for both Fanny and, with his own health failing, himself—it is an unforgettable moment of calm before the anguished finale.