We can now interpret conventional tonal cadences in functional terms:
Perfect cadences (Amer. Authentic cadences, with sub-categories "perfect" and "imperfect") involve D-T
Imperfect cadences (Amer. half cadences) involve T-D or PD-D
Phrygian cadence may be subsumed under Imperfect cadence but in view of its modal origins (i.e. if labelled as ii6-III), it becomes irrelevant/anachronistic to determine its functionality in the later tonal sense
Interrupted cadences (Amer. deceptive cadence or deceptive motion) involve D-(T)
Plagal cadences are essential T expansions
Given the possibilities of harmonic expansion which in turn imply to a hierarchical understanding of tonal harmony, cadential situations can be fascinatingly complex. Consider the following cases:
Ex. 10.4 - On surface, the piece ends with a plagal cadence across bs. 55-56; in functional terms, we hear the perfect cadence across bs. 53-54 as the structural cadence, whose tonic is expanded (plagally) over bs. 55-57.