I. COMPOSITE NATURE OF THE SONATA.
Undoubtedly the most important of all musical forms to-day is the sonata, as will easily be recognized if we remember that not only the pieces which bear this name as a title, but also the numerous symphonies, overtures, concertos, and trios, quartets, quintets, and so on, are examples of this form. The symphony is simply a sonata, on a large scale, for orchestra; the overture is a similar piece for orchestra, in one movement; the concerto is, as it were, a symphony with a solo instrument emphasized or placed in the foreground; trios, quartets, quintets, etc., are sonatas for various groups of string and wind instruments. Thus it will be seen that the bulk of all instrumental music is cast in this ever available and useful form of the sonata.
At this point, however, a confusion is likely to arise from the fact that the term "sonata" is used in two senses. It means sometimes a complete piece of music in three or more distinct movements; at other times it means a scheme or plan of musical structure exemplified in one or more of these movements, usually the first. When used in this sense it is generally coupled with the word "form": this is the way in which we shall use it here, letting "sonata-form" mean this peculiar type of musical structure, to be described in detail presently, while using "sonata" alone to name a complete composition of which one or more movements are in "sonata-form."
The sonata, as written by Philip Emanuel Bach, Haydn, Mozart, Beethoven, and modern composers, usually contains some movements in forms more primitive than "sonata-form," and already familiar to us. Thus the minuet, which often appears as the second or third movement of a sonata, has changed little since Mozart's day; the rondo, frequently used in the finale of a sonata, remains in all essentials as it is presented in the last diagram of Chapter VI; and the theme and variations, so far as its formal plan is concerned, has remained very much as Haydn left it, although, in common with the rondo, it has been vastly enriched in content and diversified in style by the genius of Beethoven.