To these latter comparatively "formless" designs belong:—all the group-forms; the majority of fantasias, the potpourri, and, as a rule, all so-called tone-poems, and descriptive (program) music generally.
On the other hand, those irregular designs which nevertheless admit of analysis according to the fundamental principles of structural logic, and are therefore directly referable to one or another of the regular forms, may be classified in the following four-fold manner—as Augmentation, Abbreviation, Dislocation, or Mixture, of the proximate fundamental design.
1. AUGMENTATION OF THE REGULAR FORM.—To this species belong those forms (small and large) which are provided with a separate Introduction, or Interludes, or an independent Coda (in addition to, or instead of, the usual consistent coda).
For example, Beethoven, pianoforte sonata, op. 13, first movement; the first ten measures (Grave) are a wholly independent Introduction, in phrase-group form, with no other relation to the following than that of key, and no connection with the fundamental design excepting that of an extra, superfluous, member. The principal theme of the movement (which is a sonata-allegro) begins with the Allegro di molto, in the 11th measure. Similar superfluous sections, derived from this Introduction, reappear as Interlude between the Reposition and Development, and near the end, as independent sections of the coda.
In a manner closely analogous to that just seen, the fundamental design of any movement in a concerto is usually expanded by the addition of periodically recurring sections, called the "tutti-passages," and by a "cadenza," occurring generally within the regular coda. In some concerto-allegros (for instance, in the classic forms of Mozart, Beethoven and others), the first orchestral tutti is a complete introductory Exposition, in concise form, of the thematic material used in the body of the movement. See the first piano-forte concerto of Beethoven, first movement.
Further, when the design is one of unusual breadth, as in some symphonic movements, or in elaborate chamber music, the number of fundamental thematic members may be so multiplied that it is necessary to assume the presence of two successive Subordinate themes, of equal independent significance,—such significance that neither of them could be confounded with a mere codetta, or any other inferior thematic member. See Beethoven, pianoforte sonata, op. 7, first movement; the Subordinate theme runs from measure 41 to 59; it is followed by another thematic section (60-93) which is so independent, important and lengthy, that it evidently ranks coordinate with the former, as second Subordinate theme. It might, it is true, be called the second Part of the Subordinate theme (the latter being no more than a repeated period); or it might be regarded as the first codetta; its thematic independence seems, however, to stamp it Second Subordinate theme.
Further, it is not uncommon to extend the sonatine-form by adding, at the end, a more or less complete recurrence of the Principal theme,—instead of, or dissolved into, the customary coda. This may be seen in Mozart, pianoforte sonata, No. 3, Andantino; the superfluous recurrence of the Principal theme begins in measure 19 from the end, after the regular sonatine-design has been achieved, fully, though concisely.
2. ABBREVIATION OF THE REGULAR FORM.—This consists chiefly in the omission of the Principal theme after the Development (that is, in beginning the Recapitulation with the Subordinate theme). Other contractions, by omission of portions (Parts) of important thematic members, during the Recapitulation, are also possible, but not so common.
An illustration of the omitted Principal theme may be found in Mendelssohn, Songs Without Words, No. 5:—
Principal Theme, period, extended (measures 1-11, dissolved into Transition—18).