400px|thumb|Main Theme of the Fantaisie-Impromptu
Frédéric Chopin's Fantaisie-Impromptu () in C minor, Op. posth. 66, WN 46 is a solo piano composition. It was composed in 1834 and published posthumously in 1855 despite Chopin's instruction that none of his unpublished manuscripts be published. The Fantaisie-Impromptu is one of Chopin's most frequently performed and popular compositions.
History
The Fantaisie-Impromptu was written in 1834, as were the Four Mazurkas (Op. 17) and the Grande valse brillante in E major (Op. 18), but unlike these other works, Chopin never published the Fantaisie-Impromptu. Instead, Julian Fontana published it posthumously, along with the waltzes Opp. 69 and 70. It is unknown why Chopin did not release the Fantaisie-Impromptu. James Huneker called parts of it "mawkish" and "without nobility". Ernst Oster conducted a technical examination of the piece that hints at similarities between the Fantaisie-Impromptu and Ludwig van Beethoven's "Moonlight" Sonata (Quasi una fantasia), which he cites as the reason for Chopin's reluctance to publish the piece. It is also recognized that it resembles the Impromptu in E major, Op. 89, by Ignaz Moscheles and published in 1834, the same year Chopin wrote the Fantaisie-Impromptu.
The mystery may have been solved in 1960 when pianist Arthur Rubinstein acquired the "Album of the Baroness d'Este", which had been sold at auction in Paris. The album contained a manuscript of the Fantaisie-Impromptu in Chopin's own hand, dated 1835, stating on the title page in French "Composed for the Baroness d'Este by Frédéric Chopin". Its authenticity was "guaranteed by the French authorities" and it shows "a delicate care for detail" and "many improvements in harmony and style" in comparison to the previously published version. Rubinstein considered it proof that it is the finished work. In his preface to the "Rubinstein Edition", published by G. Schirmer, Inc. in 1962, he surmises that the words "Composed for" in place of a dedication imply that Chopin received a paid commission for the work, so he had actually sold it to the Baroness.
Form
thumb|right|500px|Melodic fragment (introduced in measures 7-8), Chopin's Fantaisie-Impromptu
thumb|right|500px|Cadenza (measure 188), Beethoven's [[Piano Sonata No. 14 (Beethoven)|Piano Sonata No. 14, third movement]]
Oster observes that the Fantaisie-Impromptu draws many of its harmonic and tonal elements from Beethoven's Moonlight Sonata, also in C minor, especially the third movement. Two measures after the melody sets in, an abrupt run features the same notes, only one octave higher, as the cadenza in the sonata's third movement (Presto agitato). The climax on a Chord inversion| chord is similar in both pieces.