Two adjacent phrases may combine to form a period if:
- The second phrase ends with a strong cadence—usually perfect authentic. Closure (finality) must be achieved at the end of the second phrase.
- The first phrase ends with a weaker cadence than the second. A half cadence is common at the end of the first phrase.
- The two phrases bear some musical relationship to each other. Often, they will create a “question–answer” effect called antecedent–consequent. The first phrase acts as the antecedent (question) and the second phrase as the consequent (answer).
Two adjacent phrases form a parallel period if they both begin in the same manner. The two phrases may be nearly identical except for the cadences, or they may only be similar for a measure or two.
With modulation, some further information may be helpful. In two-phrase periods:
- Either phrase may contain a modulation.
- Either phrase may cadence in a key different from the key at the beginning of the period.
- The basic definition of a period remains: the cadence at the end of the second phrase must be stronger than the cadence at the end of the first phrase.
A contrasting period results when the two phrases are not similar in melodic content. The second (consequent) phrase may be different because of a change in the melodic contour or the inclusion of a dissimilar rhythmic figure, or it may simply differ in the lack of reference to material contained in the first phrase.
Although most periods are composed of just two phrases, those of three and more do occur. The three-phrase period may be organized as A A B (antecedent, antecedent, consequent) or A B B (antecedent, consequent, consequent). Whatever the relationship, the third phrase must end with a stronger cadence than either of the first two.
Double Period (Four-Phrase Period)
Sometimes known as the four-phrase period, the double period allows for a variety of phrase relationships. However, the same principle that governs two-phrase periods applies here as well: the fourth phrase must bring the period to closure and should be at least as strong as any of the other three.
Sometimes a series of phrases, some of which may be unrelated or lacking closure, do not arrange themselves conveniently into periods. Terms for such groupings range from “phrase groups” or “phrase chains” to “dissimilar phrases” or “dissolved periods.” For purposes of analysis here, these nonperiod combinations can be called dissimilar phrases.