From www.mutopiaproject.org

Frédéric Chopin wrote a number of preludes for piano solo. His cycle of 24 Preludes, Op. 28, covers all major and minor keys. In addition, Chopin wrote three other preludes: a prelude in C♯ minor, Op. 45; a piece in A♭ major from 1834; and an unfinished piece in E♭ minor. These are sometimes referred to as Nos. 25, 26, and 27, respectively.
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