Mozart’s music is generally full of joy and exuberance, but on rare occasions the composer moves into darker worlds through the use of minor keys. There just two minor-key symphonies—Symphony No. 40, perhaps Mozart’s best-known, and No. 25, whose frantic, turbulent opening bars can be heard in the first shocking minutes of the 1984 movie Amadeus. Among Mozart’s 27 piano concertos are, again, only a pair in a minor tonality, including No. 20 in D minor, written in the same key as Mozart’s powerful Requiem, left unfinished at his untimely death in 1791 at 35.