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. [Transcriber's Note: Corrected error "Sec. 3" in original.]
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.
The _rondo_ is the oldest of the larger monophonic forms and has been used in many different ways, but perhaps its most characteristic construction is as follows: (1) Principal subject; (2) second subject in dominant key; (3) principal subject; (4) third subject; (5) first subject again; (6) second subject, in _tonic key_; (7) coda (or ending).
The student should note particularly the problem of repetition and contrast (mentioned in Sec. 134) as here worked out, as the rondo was the first monophonic form in which this matter was at all satisfactorily solved, and its construction is especially interesting because it is readily seen to be one of the direct predecessors of the highest form of all--the sonata. Examples of rondos may be found in any volume of sonatas or sonatinas.