Classical Western composers structure music in much the same way writers do. Just as writers use punctuation to highlight the emotional impact of their words, composers use harmonies. One of the most essential parts of any musical phrase is the cadence, the pair of chords at the end. There are four basic types of harmonic cadences: Authentic, Half, Plagal, and Deceptive. Each delivers a unique mood.
The most common is the Authentic. This cadence, made up of a dominant chord followed by a tonic chord, offers a definitive ending. No other cadence signals a conclusion as effectively as the Authentic. This sense of stability` is created by the resolution of the unstable leading tone, contained in the dominant chord, into the tonic.
There are two types of Authentic cadence. In a Perfect Authentic cadence, both chords are in root position (V-I or V-i), and the soprano voice ends on the first scale degree. Because it sounds the most final of all Authentic cadences, it is often reserved for the very end of a piece. A dominant-tonic cadence with any other voicing is an Imperfect Authentic cadence. The dominant chord may be in a different inversion and the soprano may end on the third or fifth scale degrees. Also, the dominant chord may be replaced by the vii° chord (vii°6-I or vii°6-i). These substitutions sound less stable; composers use them to add more emotional variety than the definitive conclusion of the Perfect Authentic.
The Half cadence is the aural and functional opposite of the Authentic. It ends on the dominant chord, creating a sense of questioning. Common Half cadences are I-V, ii-V, and IV-V in major keys, or i-V, ii°6-V, and iv-V in minor keys. In the Phrygian Half cadence, a dominant chord in root position follows a minor subdominant chord in first inversion. This creates the Phrygian Half’s signature downward half step movement in the bass. However the Half cadence is composed, it always ends on the dominant chord, and its incomplete sound encourages forward momentum in the phrase.
The Plagal cadence (IV-I or, rarely, ii6-I in major keys, iv-i in minor keys) creates the distinctive “Amen” sound often heard at the end of hymns. The Plagal cadence’s effect is conclusive, but with a softer color than the Authentic cadence’s. Because of its lack of a leading tone, the subdominant does not pull as strongly to resolve into the tonic. This creates a weaker or gentler sense of resolution.
Finally, the Deceptive cadence lives up to its name. It is a dominant followed by a submediant chord (V-vi or V-VI), voiced so that the leading tone resolves to the tonic. This cadence is most effective when placed where the listener expects an Authentic cadence. The fact that the submediant shares two of the tonic chord's pitches makes it a perfect surprise to delay the end of the phrase. The resolution, usually an Authentic cadence, is much more satisfying when it does arrive.
The four types of harmonic cadences each create a unique mood. The Authentic cadence’s declarative statement is perfect for a dramatic ending, while the Half cadence creates the perfect question mark. The Deceptive cadence’s tension adds drama. The Plagal cadence provides a warmer sense of conclusion than its Authentic cousin. These harmonies provide musicians with the tools to create great varieties of drama and resolution.