and at times he did some revision to the music while editing it. Furthermore, Chopin provided his publishers with varying sources, including autographs, annotated proofsheets, and scribal copies. Only recently have these differences gained greater recognition. Form and harmony Improvisation stands at the centre of Chopin's creative processes. However, this does not imply impulsive rambling: Nicholas Temperley writes that "improvisation is designed for an audience, and its starting-point is that audience's expectations, which include the current conventions of musical form." The works for piano and orchestra, including the two concertos, are held by Temperley to be "merely vehicles for brilliant piano playing ... formally longwinded and extremely conservative". After the piano concertos (which are both early, dating from 1830), Chopin made no attempts at large-scale multi-movement forms, save for his late sonatas for piano and cello; "instead he achieved near-perfection in pieces of simple general design but subtle and complex cell-structure." Rosen suggests that an important aspect of Chopin's individuality is his flexible handling of the four-bar phrase as a structural unit. J. Barrie Jones suggests that "amongst the works that Chopin intended for concert use, the four ballades and four scherzos stand supreme", and adds that "the Barcarolle Op. 60 stands apart as an example of Chopin's rich harmonic palette coupled with an Italianate warmth of melody." Temperley opines that these works, which contain "immense variety of mood, thematic material and structural detail", are based on an extended "departure and return" form; "the more the middle section is extended, and the further it departs in key, mood and theme, from the opening idea, the more important and dramatic is the reprise when it at last comes." Chopin's mazurkas and waltzes are all in straightforward ternary or episodic form, sometimes with a coda. The mazurkas often show more folk features than many of his other works, sometimes including modal scales and harmonies and the use of drone basses. However, some also show unusual sophistication, for example Op. 63 No. 3, which includes a canon at one beat's distance, a great rarity in music. Chopin's polonaises show a marked advance on those of his Polish predecessors in the form (who included his teachers Żywny and Elsner). As with the traditional polonaise, Chopin's works are in triple time and typically display a martial rhythm in their melodies, accompaniments, and cadences. Unlike most of their precursors, they also require a formidable playing technique. The 21 nocturnes are more structured, and of greater emotional depth, than those of Field (whom Chopin met in 1833). Many of the Chopin nocturnes have middle sections marked by agitated expression (and often making very difficult demands on the performer) which heightens their dramatic character. Chopin's études are largely in straightforward ternary form. He used them to teach his own technique of piano playing—for instance playing double thirds (Op. 25, No. 6), playing in octaves (Op. 25, No. 10), and playing repeated notes (Op. 10, No. 7). The preludes, many of which are very brief (some consisting of simple statements and developments of a single theme or figure), were described by Schumann as "the beginnings of studies". Inspired by J.S. Bach's The Well-Tempered Clavier, Chopin's preludes move up the circle of fifths (rather than Bach's chromatic scale sequence) to create a prelude in each major and minor tonality. The preludes were perhaps not intended to be played as a group, and may even have been used by him and later pianists as generic preludes to others of his pieces, or even to music by other composers, as Kenneth Hamilton suggests: he has noted a recording by Ferruccio Busoni of