The large two- and three-part primary forms usually have sixteen-measure periods instead of eight-measure ones, but are otherwise similar in construction.
These various primary forms are used in constructing many varieties of compositions, among them the theme and variations, the polka, the waltz, the march, etc., as well as most of the shorter movements in sonatas, quartets, etc. They are used in vocal music also, but are less apt to be regular here because the form of vocal music is largely dependent upon the structure of the text.
147. A theme is a fragment of melody used as the subject of a fugue, as the basis of the development section in "sonata form," etc. Sometimes it is a complete tune (often in period form), on which variations are made, as e.g., in the familiar theme and variations.
[148.] Thematic development consists in taking a short theme (or several short themes) and by means of transposition, interval expansion and contraction, rhythmic augmentation and diminution, inversion, tonality changes, etc., building out of it a lengthy composition or section of a composition. Fig. 60 b, c, d, e, and f show how the theme given in Fig. 60 (a) may be varied in a few of these ways. There are hundreds of other fashions in which this same theme might be varied without destroying its identity. For other examples of thematic development see the development section of Sonata Op. 31, No. 3, as analyzed in [Appendix E].
For further illustrations of development in the case of this same theme, see—Christiani—The Principles of Expression in Pianoforte playing, p. 144, ff. from which the foregoing themes have been adapted.
149. A rondo is an instrumental composition (in homophonic style) in which a certain theme appears several times almost always in the same form (i.e., not thematically varied), the repetitions of this theme being separated by contrasting material.