From www.mutopiaproject.org

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 It was composed in 1834 and published posthumously in 1855 despite Chopin's instruction that none of his unpublished manuscripts be published.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.
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