Theme and Variations. According to some musicologists, there’s no such genre because it has no rules. Yet composers of every stripe have taken pleasure in transforming themes in any number of ways—or even, in the case of Elgar’s “Enigma” Variations, reworking variations on no stated theme at all. For keyboardists, J. S. Bach’s Goldberg Variations on an original theme he calls an aria (published in 1742) stood as the unchallenged exemplar until Beethoven knocked off his Diabelli Variations (published in 1824). Then along came Frederic Rzewski with The People United Shall Never Be Defeated (1975). Igor Levit may not have been the first pianist to regard the three works as panels of a triptych, but when he released all three as one three-and-a-half-hour boxed album, The New York Times acknowledged “an unusually high-profile (and persuasive) case for a modern work’s acceptance into the canon.” A five-star review in The Guardian noted that not even Rzewski’s own performance of The People United could “match Levit’s combination of supreme, sometimes breathtaking accuracy and his sustained intensity.” For this Carnegie Hall recital, Levit has announced only the Beethoven (based on a ditzy little waltz by a publisher in Vienna) and the Rzewski (based on a protest song from Chile), but he’s been known to play big encores. —Matthew Gurewitsch