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Frédéric Chopin Fantaisie Impromptu in C♯ minor Op. posth. 66 was composed in 1834 as were the Four Mazurkas (Op. 17) and the Grande valse brillante in E♭ major (Op. 18), and published posthumously in 1855 even though Chopin had instructed that none of his unpublished manuscripts be published. The Fantaisie-Impromptu is one of Chopin's most frequently performed and popular compositions.
Chronologically the first of Chopin’s impromptus, the Fantasy-Impromptu was not intended by the composer for publication. The assumption arose that it was withheld on account of a stated similarity – unintentional, no doubt – to the Impromptu in E flat major by Ignaz Moscheles. However, Arthur Rubinstein, who came into the possession of one of its manuscripts, assumes that this work was sold to a grande dame (the Baroness d’Est), who simply wanted to have it as her exclusive property. One way or another, the C sharp minor Impromptu remained in the composer’s portfolio and, along with the other inedita, was supposed to have been consumed by fire.
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. Also, 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."
This work has the shape of a reprise miniature, like the reverse of a form which is frequent in the nocturnes. In an impromptu, the outer sections are dominated by increased movement, while the middle section brings a dreamy lull. This shape was already familiar to Schubert; Chopin took it up and employed it also in his three later impromptus, establishing the pattern in a way that was far-removed from all schematicism. In his first impromptu, however, he could not avoid a certain schematicism. He did, though, offer a certain innovation in the area of texture. He had each of the hands play in a different metre: the right in duple time, the left in triple time. Hence the outer sections, adhering to an allegro agitato tempo, took on an almost etude-like appearance (bars 5–12). The middle section brings music that is typical of the slow sections of nocturnes. It proceeds at a moderato cantabile tempo, spinning out sotto voce a melody in D flat major, which returns a couple of times, as i
