Overview

The concerto is divided into the following three movements:^[2]

The concerto is scored for one flute, two oboes, two clarinets, two bassoons, two horns, two trumpets, timpani and strings.^[2] This is the largest array of instruments for which Mozart composed any of his concertos.^[12]

It is one of only two of Mozart's piano concertos that are scored for both oboes and clarinets (the other, his concerto for two pianos, has clarinets only in the revised version). The clarinet was not at the time a conventional orchestral instrument. Robert D. Levin writes: "The richness of wind sonority, due to the inclusion of oboes and clarinets, is the central timbral characteristic of [the concerto]: time and again in all three movements the winds push the strings completely to the side."^[5]

The solo instrument for the concerto is scored as a "cembalo". This term often denotes a harpsichord, but in this concerto, Mozart used it as a generic term that encompassed the fortepiano, an eighteenth-century predecessor of the modern piano that among other things was more dynamically capable than the harpsichord.^[13]

I. Allegro

The first movement is longer and more complex than any that Mozart had previously composed in the concerto genre.^[14] It is in ³
₄; among Mozart's 27 piano concertos, No. 4 in G Major, No. 11 in F major and No. 14 in E♭ major are the only others to commence in triple metre.^[2]

The first movement follows the standard outline of a sonata form concerto movement of the Classical period. It begins with an orchestral exposition, which is followed by a solo exposition, a development section, a recapitulation, a cadenza and a coda. Within this conventional outline, Mozart engages in extensive structural innovation.^[15]

II. Larghetto

Alfred Einstein said of the concerto's second movement that it "moves in regions of the purest and most moving tranquility, and has a transcendent simplicity of expression".^[31] Marked Larghetto, the movement is in E♭ major and cut common time. The trumpets and timpani play no part; they return for the third movement.^[32]

The movement opens with the soloist playing the four-measure principal theme alone; it is then repeated by the orchestra.

This theme is, in the words of Michael Steinberg, one of "extreme simplicity".^[33] Donald Tovey refers to the fourth bar, extremely bare and lacking any ornamentation, as "naive", but considers that Mozart intended for it to be so.^[25] Mozart's first sketch of the movement was much more complex. He likely simplified the theme to provide a greater contrast with the dark intensity of the first movement.^[34] After the orchestra repeats the principal theme, there is a very simple bridge or transitional passage that Girdlestone calls "but a sketch" to be ornamented by the soloist, arguing that "to play it as printed is to betray the memory of Mozart".^{[35][lower-alpha 2]}

Following the bridge passage, the soloist plays the initial four-measure theme for a second time, before the orchestra commences a new section of the movement, in C minor. A brief return of the principal theme, its rhythm altered,^[33] separates the C minor section from a section in A♭ major.^[36] After this new section, the principal theme returns to mark the end of the movement, its rhythm altered yet again.^[33] Now, the theme is played twice by the soloist, the two appearances being connected by the same simple bridge passage from the beginning of the movement. Girdlestone argues that here "the soloist will have to draw on his imagination to adorn [the simple bridge passage] a second time".^[35] The overall structure of the movement is thus ABACA, making the movement in rondo form.^[37]

In the middle statement of the principal theme (between the C minor and A♭ major sections), there is a notational error which, in a literal performance of the score, causes a harmonic clash between the piano and the winds. Mozart probably wrote the piano and wind parts at different times, resulting in an oversight by the composer.^[38] Alfred Brendel, who has recorded the concerto on multiple occasions, argues that performers should not follow the score literally but correct Mozart's error. Brendel further argues that the time signature for the whole movement is another notational error: played in cut common time, which calls for two beats per bar rather than four, the movement is, in his view, too fast.^[39]

The form of the movement is nearly identical to that of the second movement of Mozart's Piano Sonata in B♭ major, K. 570.

III. Allegretto

The third movement features a theme in C minor followed by eight variations upon it.^[40] Hutchings considered it "both Mozart's finest essay in variation form and also his best concerto finale."^[41]

The tempo marking for the movement is Allegretto. Rosen opines that this calls for a march-like speed and argues that the movement is "generally taken too fast under the delusion that a quick tempo will give it a power commensurate with that of the opening movement."^[29] Pianist Angela Hewitt sees in the movement not a march but a "sinister dance".^[7]

The movement opens with the first violins stating the theme over a string and wind accompaniment. This theme consists of two eight-measure phrases, each repeated: the first phrase modulates from C minor to the dominant, G minor; the second phrase modulates back to C minor.^[42] The soloist does not play any part in the statement of the theme, entering only in Variation I. Here, the piano ornaments the theme over an austere string accompaniment.^[43]

Variations II to VI are what Girdlestone and Hutchings independently describe as "double" variations. Within each variation, each of the eight-measure phrases from the theme is further varied upon its repeat (A_XA_YB_XB_Y).^{[40][43][lower-alpha 3]} Variations IV and VI are in major keys. Tovey refers to the former (in A♭) as "cheerful" and the latter (in C) as "graceful".^[44] Between the two major-key variations, Variation V returns to C minor; Girdlestone describes this variation as "one of the most moving".^[45] Variation VII is half the length of the preceding variations, as it omits the repeat of each eight-measure phrase.^[43] This variation concludes with an extra three-measure passage that culminates in a dominant chord, announcing the arrival of a cadenza.^[46]

After the cadenza, the soloist opens the eighth and final variation alone, with the orchestra joining after 19 measures. The arrival of the final variation also brings a change in metre: from cut common time to compound duple time.^[47] Both the final variation and the coda which follows contain numerous neapolitan-sixth chords. Girdlestone referred to the "haunting" effect of these chords and stated that the coda ultimately "proclaims with desperation the triumph of the minor mode".^[46]