A phrase is the smallest structural unit that builds and releases tension and the main means through which it achieves its tension release is a single cadence.
Cadences delimit phrases and this is the reason why cadences are important in determining the structure of compositions. Most often, a cadence occurs at the end of a phrase. Sometimes, additional material is added after a cadence. For instance, a plagal cadential progression can appear after a perfect authentic cadence to add more weight to the cadence.
Phrases originates from vocal music. In vocal music, a singer cannot sing too long without taking a breath. Even in instrumental compositions, phrases are usually focused around presenting melodic ideas and the length of a phrase is limited. Often, a phrase is made of four measures, or two measures in a slower tempo, or eight measures in a faster tempo. Usually, most of the phrases in a composition have the same number of measures. In this case, we can say that a composition makes a normative for the length of its phrases. This affects listeners’ expectations. However, composers also include phrases of different lengths to surprise listeners and make a composition more interesting. Such a phrase can be longer or shorter than expected.
It is not always easy or even possible to find phrases in material. It depends on character of material. Material can be more or less thematic. Thematic material is focused around presenting melodic ideas and it is recognizable, memorable and singing. In such a material, closing gestures are used in a predictable way and are easy to perceive. Thus, it is easy to find cadences and phrases. Nonthematic material has characteristics of freedom and continuous flow. In such a material, it is hard to perceive closing gestures. Thus, it is hard to find cadences and phrases.
Usually, a phrase is not independent. That is, it depends on the previous or the next phrase. In this case, the adjacent phrases form a structure unit called the period or a bigger structure unit called the phrase group. This will be explained in the next chapter. Two adjacent phrases can be in different time relationships. The next phrase can start immediately after the previous one or there can be some short material between them which is used to smoothly link the end of the previous phrase to the beginning of the next one. Sometimes, phrases overlap i.e. the end of the previous phrase is simultaneously the beginning of the next one.
Examples of phrases, together with their cadences, will be examined in the next chapter and chapter Form and phrases analysis.