The slow movement is ultimately derived from the folk-song, and, while more subject to operatic influence than were the other movements, it still retains something of that simple lyric quality that distinguished it in its primitive form. Unlike the other movements of the sonata and symphony, however, the slow movement has no settled form: _i. e._, while we speak of first-movement, or sonata-form, of the rondo form, and of the minuet form, we do not speak thus of "slow movement form." For in the slow movement style rather than form is of greatest importance. On account of its slow tempo it is shorter than the first movement, and consequently not so dependent for intelligibility on formal structure. Its themes, also, are song-like in character, and song themes, being in themselves complete, do not lend themselves readily to development--do not generate new material--as has already been pointed out. As a consequence the slow movement is usually written in what we call a "sectional" form: _i. e._, a series of sections following one another according to whatever order or system the composer may choose. The most common use is, however, the form employed in the minuet. But in slow movements the long song themes, somewhat elegiac in style and full of sentiment, make the _mood_ of each section of supreme importance, and throw the formal element into the background. So that, while the slow movement usually falls under some one of the common forms already discussed, it often modifies them in one way or another.
There are rare instances of developed ternary form in the slow movements of Mozart's pianoforte sonatas. The Andante of the Sonata in B-flat (no. 10 in Schirmer's edition), has a development section. It comprises only nineteen measures, however, and its effect as a section germinating from the exposition is somewhat lessened by the scheme of repeats, which is as follows: A :⎜⎜: B. A. :⎜⎜. The use of rondo form in the slow movement will be discussed in a later chapter.
II. SLOW MOVEMENTS OF PIANOFORTE SONATAS.
Reference has already been made in Chapters VI and VII to the lack of sustaining power in the tone of the pianoforte of Haydn and Mozart's day, and the consequent use of ornament in their pianoforte music. In Figure XLI (_a_) is shown the beginning of the andante of Mozart's sonata referred to above, and at (_b_) will be found the corresponding portion of the restatement in the same movement. These two quotations should be compared with the corresponding portions of the two pieces that serve as examples for analysis with this chapter. This comparison will reveal how much more highly ornamented was the music written for the piano than that for instruments with sustained tone.