The slow movement is of all the parts of the sonata the least uniform in structure. Often it is written in the primitive aria-form, identical with the minuet form; sometimes it is an adaptation of rondo form to the exigencies of deliberate movement and emotional eloquence; and not infrequently it is a modification of “first movement form,” or sonata form proper. Its value depends but little on its structure, and almost entirely on its expressive qualities. Of all the movements of the sonata, as has already been said, the first, which comes when the listener is fresh and disposed to give minute and unflagging attention, is the most complex. First movement form, however, is but a further application of the simple principles of statement, contrast, and reassertion, and of contrast of keys, that are already operative, in an easily understood way, in the minuet, the aria, and the rondo. The first movement of a regular sonata begins with a first subject, or theme, in the tonic key, built, of course, upon a striking, individual, and memorable motif. After this has been well impressed upon the mind by a certain amount of repetition, either literal or modified, there is a formal transition to a contrasted key, generally the dominant, or, if the movement be in minor, the relative major, and a second subject enters, is in its turn well impressed upon the attention, and ends with an emphatic cadence or close in the contrasted key. This much makes up one complete section of the form. Historically, it is an outgrowth of the first part of an ordinary small ternary form, by simple magnification of the elements, and increasing definition of and contrast between them. What was at first but an inconspicuous modulation becomes a formal transition; and what was but a cadence in the contrasting key becomes a new subject, with its own individuality and function in the organism. And thus is built up the section of statement, with quite a high degree of complexity of its own. This is sometimes called the Exposition.