THE SONATA-ALLEGRO FORM
THE works of Domenico Scarlatti, Galuppi, Paradisi, Kuhnau, and C. P. E. Bach brought about a gradual development of the sonata, until it took shape in the hands in Haydn and Mozart. The distinctive quality of the sonata consists in the shape of its first movement; and as this first movement is generally an allegro, the shape is mentioned here as sonata-allegro form. The sonata consists of various movements, whose contrasts of style admit of much artistic excellence. The same excellence is found in the form used in the first movement.
The sonata-allegro form is first of all divided into three main parts. These are the exposition, or first playing of the themes used; the development, or building up of a tonal structure from the material in the themes; and the recapitulation, or return of themes. If desired, a middle part, of new material, may be substituted for the development; but composers do not usually make this substitution without some good reason.
The themes used in a sonata may be much freer than those of a song-form. It is this variety of material in sonata themes that makes the piano sonatas of Beethoven so great. There is practically no limit to the power and expression that the composer may put into such themes.
The principle of contrast is introduced in the exposition; for the themes, three in number, may be of different styles. In general, the chief theme, or first theme, is expected to be bold and resolute in character, while the second theme should be more lyrical and tender. Between the two is a short tributary passage, of modulatory character. After the second theme comes a short closing theme, usually of brilliant style. The exposition is always marked for repeat, so that the themes may be clearly suggested to the hearer; but the modern tendency is to do away with many repeats. In piano sonatas the exposition is still usually given twice, though in symphonies the repeat is optional with the conductor.