Frédéric Chopin’s Fantaisie-Impromptu in C-sharp minor, Op. posth. 66, is a solo piano composition. It was composed in 1834 and dedicated to Julian Fontana, who published the piece in spite of Chopin’s request not to do so.
Some aspects of this piece are similar to Beethoven’s Moonlight Sonata, which is also in C-sharp minor. Two measures after the melody begins, an abrupt run up and down has exactly the same notes as the cadenza in movement 3 (Presto agitato) of that work. The climax on a six-four chord is similar in both pieces. Also, the Fantaisie-Impromptu’s middle part and the second movement of the Moonlight Sonata are in D-flat major. The first and third movements are in C-sharp minor.
For those reasons, and many others, Felix Salzer writes, “Chopin understood Beethoven to a degree that no one who has written on the C♯ minor Sonata or the Fantaisie-Impromptu has ever understood him. … The Fantaisie-Impromptu is perhaps the only instance where one genius discloses to us — if only by means of a composition of his own — what he actually hears in the work of another genius.”
The piece uses many cross-rhythms (the right hand plays sixteenth notes against the left hand playing triplets) and a ceaselessly moving note figuration and is in cut time (2/2). The opening tempo is marked allegro agitato. The tempo changes to largo and later moderato cantabile when the key changes to D-flat major, the enharmonic equivalent of the more obscure tonic major key of C-sharp major, that is, the parallel major of C-sharp minor.
The piece then changes to presto (although some versions of the score incorporate a coda, meaning that the original tempo of allegro agitato is repeated) where it continues in C-sharp minor as before. It concludes in an ambiguous fantasy-like ending, in a quiet and mysterious way, where the left hand replays the first few notes of the moderato section theme, while the right hand continues playing sixteenth notes (semiquavers). The piece resolves and gently ends on a C-sharp major rolled chord.