Fantaisie-Impromptu

Frédéric Chopin's Fantaisie-Impromptu () in C minor, Op. posth. 66, 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 It was composed in 1834 and published posthumously in 1855 despite Chopin's instruction that none of his unpublished manuscripts be published.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 other 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 which 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 composed by Ignaz Moscheles and published in 1834, the same year Chopin composed his "Fantasie-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". The facts of its authenticity having been "guaranteed by the French authorities" and that it shows "a delicate care for detail" and "many improvements in harmony and style" in comparison to the previously published version, Rubinstein considered absolute proof that it is the finished work. In his preface to the "Rubinstein Edition", published by G. Schirmer, Inc. in 1962, Rubinstein 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.
Ernst Oster observes that the "Fantaisie-Impromptu" draws many of its harmonic and tonal elements from Beethoven's "Moonlight Sonata", which is also in C minor, and from the third movement in particular. Two measures after the melody sets in, an abrupt run features the same notes, only one octave higher, like the cadenza in the sonata's third movement (Presto agitato). The climax on a chord is similar in both pieces. Additionally, the "Fantaisie-Impromptu"s middle part and the second movement of the "Moonlight Sonata" are in D major. The first and third movements are in C minor.
For those and other reasons, Ernst Oster 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 (). The opening tempo is marked "allegro agitato". The tempo changes to "largo" and later "moderato cantabile" when the key changes to D major, the enharmonic equivalent of the more obscure tonic major key of C major, that is, the parallel major of C 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 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 major rolled chord (a Picardy third).
Legacy.
The melody of the "Fantaisie-Impromptu"s middle section was used in the popular Vaudeville song "I'm Always Chasing Rainbows". That theme was quoted in Variation 10 of Federico Mompou's "Variations on a Theme of Chopin", which is otherwise based on Chopin's Prelude No. 7 in A major. George Crumb's Makrokosmos, Volume 1: 11. Dream Images (Love-Death Music) (Gemini) includes three quotations from the "Fantaisie-Impromptu"s middle section.