Speaking broadly we may say that form in any art consists in the placing together of certain parts in such relations of proportion and symmetry as to make a unified whole. In music this implies unity of tonality and of general rhythmic effect, as well as unity in the grouping of the various parts of the work (phrases, periods, movements) so as to weld them into one whole, giving the impression of completeness to the hearer.
133. The primal basis of form is the repetition of some characteristic effect, and the problem of the composer is to bring about these repetitions in such a way that the ear will recognize them as being the same material and will nevertheless not grow weary of them. This is accomplished by varying the material (cf. thematic development), by introducing contrasting material, and by choice of key.
[134.] The student should note at the outset of this topic the difference in meaning between the terms form and style: A form is a plan for building a certain definite kind of composition, but a style is merely a manner of writing. Thus e.g., the fugue is a form—i.e., it is a plan, which although capable of variation in details, is yet carried out fairly definitely in every case; but counterpoint is merely a style or manner of writing (just as Gothic architecture is a style of building), which may be cast into any one of several forms.
135. The material found in the following sections is an attempt to explain in simple language certain terms relating to forms and styles which are in common use; in many cases the definition is too meagre to give anything but a very general idea, but it is hoped that the student will at least be set to thinking and that he will eventually be led to a more detailed and scholarly study of the subject. (The article "Form" and the separate articles under each term here defined, as found in Grove's Dictionary, are especially recommended. For examples of the various forms described, see also Mason and Surette—"The Appreciation of Music," Supplementary Volume.)
136. In a very general way there may be said to be two styles of musical composition, the monophonic (or homophonic)—the one-voiced—and the polyphonic—the many voiced. The polyphonic[32] style antedates the monophonic historically.
137. In monophonic music there is one voice which has a pronounced melody, the other voices (if present) supporting this melody as a harmonic (and often rhythmic) background. An example of this is the ordinary hymn-tune with its melody in the highest part, and with three other voices forming a "four-part harmony." The sonata, symphony, opera, modern piano piece, etc., are also largely monophonic, though polyphonic passages by way of contrast are often to be found.
138. In polyphonic music each voice is to a certain extent melodically interesting, and the "harmony" is the result of combining several melodies in such a way as to give a pleasing effect, instead of treating a melody by adding chords as an accompaniment or support. Counterpoint, canon, round, fugue, etc., are all polyphonic in style. The word contrapuntal is often used synonymously with polyphonic.
(Sections [139] to [143] relate especially to terms describing polyphonic music.)
[139.] Counterpoint is the art of adding one or more parts or melodies to a given melody, the latter being known as the "cantus firmus," or subject. It may therefore be broadly defined as "the art of combining melodies."
The word counterpoint comes from the three words "punctus contra punctum," meaning "point against point." The word point as here used refers to the punctus—one of the neumae of the mediaeval system, these neumae being the immediate predecessors of modern notes.