37. Polyphonic styles of composition are divided into three classes, distinguished chiefly by the degree of severity with which the principles of polyphony, or contrapuntal writing, are applied; namely: the Invention, the Fugue, and the Canon.
In the Canon the utmost rigor of thematic treatment prevails. In the Fugue, the contrapuntal methods are applied with less insistence, but with reference to certain specific conditions, peculiar to this class. In the Invention, the polyphonic principles (Imitation, etc.) are applied in a general way only, with much freedom of detail; and with almost, if not quite, equal regard of the broader considerations of form (structural design), and of general external effect.
The “Invention” (a term adopted from one of the musical practices of the 17–18th centuries) is, then, the simplest and least constrained of the three principal classes of Polyphonic Form.
38. The factors which enter into the construction of an Invention are: (1) The Motive; (2) The Imitations or recurrences of the Motive; (3) The Contrapuntal Associates; and (4) The Episodic passages (including the Cadences).
a. The Motive is a brief melodic sentence, calculated to be (and adopted as) the basis and source of the evolution, construction (or “invention”), of the polyphonic composition in view. It is the “theme” of the structural task.
To this end, it should be simple and regular in its harmonic design, and faultless in melodic formation; but, at the same time, sufficiently characteristic and suggestive to invite manipulation, and to sustain interest. For this reason, somewhat striking melodic features may be admitted, and moderate rhythmic irregularities are appropriate, if not necessary, — hence the frequency of the tie, in effective Motives.
Review, very minutely, as a test of these details, the formation of all the Motives given in Exercise 6B (2–8) and Exercise 7A.
The length of such a thematic sentence concerns mainly the theoretical distinction made between
Some of the “themes” of Exercises 6 and 7 are, properly speaking, Subjects, though the generic term “Motive” will be applied to all alike, for convenience. Par. 113 may be briefly referred to.
The most important rule for the Motive is, that its first tone, or tones, should impart an unmistakable Tonic impression: —
See Exercise 6B, Nos. 2, 3, 6, 8, — beginning with the Tonic note; also Exercise 7A, Nos. 1, 4, 5, 6, 7, 9, 12, 13, 14, 15, 21, 23, 24. Further, Exercise 7A, Nos. 2, 3, 8, 10, 11, 17, 22, 26, — beginning with the Dominant note, followed immediately, or soon, by the Tonic. In Exercise 7A, Nos. 19 and [no.] 25, the Tonic note follows quite late, but the Tonic impression is clear. Exercise 7A, No. 18, is misleading, as the Tonic note is entirely absent. In Exercise 6B, Nos. 5 and [no.] 7 begin upon the Mediant, followed soon by the Tonic, — this is perfectly clear. Exercise 7A, No. 16, is again misleading, because of the long absence of the Tonic, after the initial Mediant (which imparts the impression of e minor, instead of G major). Exercise 6B, No. 4, begins with the lower neighbor of the Tonic, and is entirely clear. Exercise 7A, No. 20, is misleading, because the upper neighbor of the Mediant, though unaccented, imparts a B♭ major impression where F major is intended.
b. The Imitations, or successive recurrences of the Motive, constitute the so-called thematic texture of the polyphonic fabric. Compare par. 38d, and review Ex. 69, Note *2). The details of this factor are given in Chapter IV.
c. This term is applied (possibly a little loosely) to all the passages that are invented as contrapuntal associates of the motive, upon each of its successive Imitations or recurrences See par. 34.
d. To all those portions of the polyphonic fabric where the motive is not present in either part, or is represented by only one of its fractional figures, the term Episodic passage is applied. The necessity for such elements in effective polyphony, as a foil to the thematic portions, is obvious; for the monotony of persistent motive-imitation must be relieved from time to time. But it is equally manifest that episodic passages should, as a rule, be neither too frequent nor too extended; and that they must be in close keeping with the character of the thematic portions.
For the latter reason, the episodes are usually derived (1) directly from the motive itself, by using some fraction of the same; or (2) indirectly, by utilizing the whole or some portion of any of the contrapuntal associates of the motive; or, possibly (3), an entirely new figure may be adopted, on condition that it be conceived strictly in the spirit of the motive, or of the general context, and that it reappear at intervals during the composition. Finally (4), it is also possible that an episodic passage, after beginning in touch with the motive, may, by gradual and consistent stages, develop into a more or less independent character, — though it can scarcely be justified in becoming quite foreign. For illustrations of the episodic components, see Exs. 89, [ex.] 90, [ex.] 91.