Frédéric Chopin

Impromptu is a term that has little meaning. A composition for piano (or, occasionally, another solo instrument) that suggests improvisation, an impromptu is often described as a work that results from sudden inspiration. Chopin's four famous Impromptus are in ternary form (ABA). Similarities between the four works have led some writers to suggest that Chopin thought of the four as a "set" in which each successive impromptu is derived, in an improvisatory fashion, from the previous one. The fourth of these, Op. 66, was the first composed and had the prefix "Fantasie" added by the editor to Chopin's "Impromptu" title.

Dedicated to the Countess Esterházy, the Impromptu in G flat major, Op. 51, was composed in 1842 and published in 1843 in Leipzig. Unlike Chopin's other impromptus, that in G flat major is in a compound meter (12/8). Opening with a single line, the piece's texture grows increasingly dense throughout the first section. The similarity between the opening and the first theme of the Impromptu in A flat major, Op. 29, is apparent from the outset. Without repeating the first theme, Chopin moves on to the second, which begins with a rising scale and closes after a long chromatic descent. The subsequent repeat of this phrase is so varied and passes through such distant harmonies that it sounds like new material. Chopin skips the first two measures of the opening theme at the return; the first hint that these measures are actually introductory and not part of the theme.

For the opening of the B section, Chopin juxtaposes two measures of constant eighth note rhythm with two measures of slower moving chords. This four-measure complex happens twice, with some variation, before giving way to a new idea featuring scales that climb to the stratosphere. The second part of the trio is in 4/4 meter, but this is not immediately apparent because Chopin continues to write triplets for the right hand. Once the duplet eighth notes in the left hand create a polyrhythmic texture, however, the new meter becomes audible. The leaping tune and polyrhythm, emphasizing the dominant (D flat), eventually dissolve into a return of the first theme, which again is missing the introductory measures. Both section A themes follow in their original forms, as does the first part of section B.