As suggested earlier, as we begin to consider the slower pulse streams of a metric structure, we become involved with melody and the grouping of melodic segments into musical phrases. What follows is a description of some prototypical formal organizations that are the basis for many musical forms.
For practical purposes, we will define a phrase as a cohesive musical gesture that ends in a cadence. We will hear many musical gestures that do not have cadential closure; these may be termed phrase units. Phrases can vary widely in length. Though they are often written over the notated space of four measures, it is wise to study the music carefully to see where the cadence occurs. Phrases of two, six and eight measures are also heard frequently. Once again, tempo is a factor: two-measure phrases are more likely to be found in slower tempos, like Largo or Grave, while eight-measure phrases are more likely to be found in faster tempos, like Vivace or Presto. Phrases of odd-numbered lengths—3, 5, and 7 measures or more—can often be understood as expansions or contractions of even-numbered units.
A period, in its most basic shape, is a pair of phrases, often eight measures in length. The most common period structure has its first phrase ending with a half cadence and its second phrase ending with an authentic cadence. When the thematic material is similar in both phrases, we apply the terms antecedent to the first phrase and consequent to the second phrase; otherwise, they may be referred to as forephrase and afterphrase, respectively.*
*The terminology used in the two paragraphs above is taken from Phrase Rhythm in Tonal Music by William Rothstein.
These thematic and cadential characteristics set up reciprocal gestures that are fundamental to most tonal musical designs. It is as if you extended your left hand forward for the first phrase, and your right hand forward for the second phrase—a kind of "THIS then THAT" gesture. Motives and fragments within phrases, phrases themselves, and larger groupings within sections will be heard in this characteristic “this and that” movement. Composers vary this reciprocal gesture in many ways; they may repeat one or both phrases, add contrasting material (including other complete phrases!), change the cadence relationships, and stretch or contract different parts of the gesture. In locating musical periods, it is often better to look for the strongest cadences first, and then sort out preceding material to determine period forms.
A sentence is also a period of eight measures; however, it has special characteristics that set it apart from the antecedent/consequent type.. The first phrase is typically composed of two clear subphrases. The second phrase usually contains fragments of the original subphrase. If we assume an eight-measure length for the sentence, the proportions will be as follows:
Sentence: 8 measures ______________________________________________________
Phrase I: 4 measures _____________________ Phrase II: 4 measures ________________
subphrase 1_________subphrase 2__________“liquidation” of Phrase I______________
2 measures 2 measures 4 measures (or 1, 1, 1, 1)
This proportion, 2 : 2 : 4, may be played out over 4-measure, 8-measure, or 16-measure spans. The term “liquidation” refers to the “condensing” and “intensification” of the original basic idea by compressing it in the second phrase.*
*The definition of the sentence, above, and the examples to follow, are taken from Arnold Schönberg’s Fundamentals of Musical Composition (St. Martin’s Press, New York:1970).
The next several examples show a variety of treatments of phrases, periods, and sentences. Some follow the models closely, other show considerable divergence. Examine and listen to them carefully.
Ex. 57 Phrase, Period, and Sentence Excerpts from Schoenberg’s Fundamentals of Musical Composition (not supplied).