203. The most frequent cadence in harmony is that involving the chord on I preceded by the chord on V. Because of its directness the cadence V—I is called the authentic cadence.
204. The most satisfactory form (to the ear) of the authentic cadence is that in which the highest voice (the soprano) of the final chord is the root of that chord. When the final chord appears in this position the cadence is called perfect[36] authentic, and when the third or fifth of the chord appear in the soprano, the cadence is called imperfect authentic. Fig. 68 shows the chord G—B—D cadencing to C—E—G in three different ways. The first one (a) is called a perfect authentic cadence, but the last two (c) and (d) are imperfect authentic.
205. A plagal cadence is one in which the tonic chord is preceded by the sub-dominant chord (IV—I). The plagal cadence (sometimes called the church cadence, or amen cadence), like the authentic, is described as being perfect when the soprano of the tonic chord is the root of that chord, and imperfect when the soprano of the final chord is the third or fifth of that chord. Fig. 69 shows the chord F—A—C cadencing to C—E—G in three ways. The first one (a) is called a perfect plagal cadence, the last two are imperfect plagal.
206. A half-cadence occurs when the dominant chord is used as the final chord of a phrase, and is immediately preceded by the tonic chord. This form is used to give variety in the course of a composition, but is not available at the end because it does not give a definite close in the tonic key. Fig. 70 shows the use of the half-cadence at the close of such a phrase.
