There are at least 3 known versions of the Fantaisie Impromptu.
The first one is the 1834 version published in 1855 by Julian Fontana. Of the 1834 version, 3 different first editions–the French Meisonnier edition, the German Schlesinger edition, and the English Ewer edition were printed.
A copy belonging to Chopin’s sister had it named as Fantaisie, for which it was likely known during his lifetime(Source: Chopin: Impromptus and Fantasias for Piano published by Stainer & Bell), while other versions name it as an Impromptu. Fontana, after being given rights to publish the music, probably decided to combine it together and label it as a “Fantaisie-Impromptu”
The 1834 manuscript has now been lost—or it may have never existed in the first place, being based off Fontana’s own copies instead.
A second version was published in 1962 based off a 1835 manuscript from the collection of Baroness Sarah Frances d’Este after being purchased by the legendary pianist Artur Rubinstein in 1955. The surviving manuscript is now in the collection of the Chopin Museum in Poland. This version is oftentimes referred to as the Final Version or Rubinstein Edition, and is considered the most authentic version.
A third version based off the 2 almost identical handwritten copies of Auguste Franchomme dated January 1849 has also been published. One copy can be found in Paris, France within the Bibliothèque du Conservatoire national de musique. The other is in the Chopin Museum in Poland.
There are also hybrid versions that have been printed over the years based on a combination of the versions available, as well as a variant for Bar 72 found in Klindworth’s edition.
There are multiple small differences in the versions.
Most markedly, a metronome mark of 84 minims to the minute is seen in the 1834 version but not the others.
The 1834 version shows thumb accents in Bars 13-16 and little finger accents in Bars 17-21, but there are none in the 1835 manuscript. Also, the first 4 RH notes differ at the start of Bars 32, and LH chords in Bars 35-36.
The version most often played today is the 1834 Fontana Version, as compared to the 1835 Final/d’Este/Rubinstein Version. The Fontana Version can be freely downloaded but the Final Version(first published after 1955) may not yet be public domain in certain countries.
1834(Bar 33 RH first 4 notes, Bar 35-36 LH)
1835(Bar 33 RH first 4 notes, Bar 35-36 LH)
This is the 1834 version, featuring Klindworth’s ossia at 2m48s.
This is the 1834 version first published in 1855 by Fontana. It is by far the most commonly played version nowadays, and the one most easily found on the public domain for free.
This is the 1835 “Final” version first released and recorded by Rubinstein in the 1960s.
Today, the piece is generally considered diploma level, although it was one of the inaugural ABRSM Grade 8 pieces back in 1933. Its last ABRSM Grade 8 appearance was in 1965, as can be seen from the scan of the exam book I procured below. Currently, it can be found on the Trinity College London ATCL associate diploma list.
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Below are links to Schlesinger’s First Edition and Klindworth’s variant which are known to be public domain works. I have also added a link to the 1835 version on IMSLP, which may or may not be public domain depending on your country. More versions are available at IMSLP. Please contribute to them if you can.
1834 versions:
1835 version: