Mendelssohn, Presto agitato in B minor for pianoforte (preceded by an "Andante cantabile" which has no connection with the sonatine-form of the presto, but may also be analyzed). This design is very broad; each factor is expanded to its fullest legitimate extent, especially the "codetta" section.
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CHAPTER XVII. THE SONATA-ALLEGRO FORM.
ORIGIN OF THE NAME.—The fully developed Sonata-allegro form is the design in which the classic overture and the first movement of the symphony, sonata and concerto are usually framed. The student must be careful not to confound this musical form with the complete sonata of three or four movements. It is not to be called the "sonata form," but the "sonata-allegro form." It is to one movement only, generally the first one, which is (or was) very commonly an allegro tempo in the sonata and symphony, that the present design refers; and its name, sonata-allegro, is derived from that old historic species of the sonata which consisted originally of but one movement, generally an allegro.
THE SONATA-ALLEGRO FORM.—As distinguished from the sonatine-form, with its two Divisions, this larger species, based upon precisely the same structural idea, has three Divisions,—the Exposition, a middle Division called the Development (growing out of the brief interlude of the sonatine-form), and the Recapitulation. The diagram (the keys of which correspond to the plan of Beethoven, op. 14, No. 2, first movement) is as follows:
| Exposition | Middle Div. | Recapitulation | ||||
| Pr. Th. | Sub. Th. | Codetta | Development, various keys, ending with Retransition | Phr. Th. | Sub. Th. | Codetta and Coda. |
| G maj. | D maj. | D maj. | G maj. | G maj. | G maj. | |
Compare this diagram, also, with that of the Third Rondo-form, and note, accurately, the points of resemblance and contrast.
Compare it, further, with the diagram of the sonatine-form, on page 122. It will be observed that here the Recapitulation does not follow the Exposition at once, as there, but that a complete middle division intervenes, instead of the brief interlude or re-transition; from which the student may conclude that the sonatine-form gradually grows into the sonata-allegro form, as this interlude becomes longer, more elaborate, and more like an independent division of the design. Or inversely, and perhaps more correctly, the sonata-allegro becomes a sonatine-design by the omission (or contraction) of the middle Division.
THE EXPOSITION.—The presentation of the thematic factors, the statement or Exposition of the two themes and codetta, is made exactly as in the sonatine-form, though probably upon a broader scale. The Principal theme is usually a Two-Part Song-form, at least; often Three-Part. In broader designs, a separate transitional passage appears; in more concise designs, the transition is developed out of the last Part of the Principal theme by the process of dissolution—as will be seen. The object of the transition is, as usual, to lead into the new key (of the Subordinate theme). It is sometimes, though very rarely, omitted.