Definition and purpose of cadences
Different degrees of the seven-tone scale have different functions. The first degree of the scale, tonic, is a stage of rest. A chord of the first degree of the scale starts and ends harmonic progressions. It is the most stable chord. Sometimes a chord of the sixth degree is used as an alternative to a chord of the first degree. However, the strength of its harmonic function is weaker. A chord of the fifth degree of the scale, dominant, has the second importance because the dominant chord has the leading tone that must be resolved to the tonic. And by the same reason, a chord of the seventh degree can be used instead of a chord of the fifth degree. The subdominant is less stable than the tonic and less dissonant than the dominant. It can be used as a preparation to the dominant chord. A chord of the fourth degree can be replaced by a chord of the second degree of the scale. The primary chords of the first, the fourth and the fifth degree are the best representation of the scale. Harmonic functions of these chords create movement in the music, moments of tension and relaxation. By using functional harmony composers write musical phrases. Cadences close phrases, sections and entire musical compositions. Some cadences mean a complete stop and others mean a momentary pause. Here are several types of harmonic cadences: authentic cadence; plagal cadence; half cadence; deceptive cadence.
An authentic cadence is a final cadence which brings the music to the conclusion, to the filling of finality and rest by resolving the dominant chord to the tonic chord: V-I, V-i, vii6-I, vii6-i, V7-I, V7-i. It is the most obvious-sounding cadence. There are two types of authentic cadence: perfect authentic cadence (PAC) and imperfect authentic cadence (IAC).
Perfect authentic cadence
In perfect authentic cadence both chords must be in the root position, the tonic chord must be on the strong beat of the meter and the tonic note must be in soprano - the highest voice. Perfect authentic cadence is the strongest cadence of all.
Imperfect authentic cadence
If the tonic note is not in the highest voice or the vii chord is substituted to V, one or both chords are inverted, the tonic chord is not on the strong part of the meter, the cadence is imperfect and it's weaker than the perfect authentic cadence.
Half cadence ends with the V chord and it is the point of tension: I-V, IV-V, ii-V, i-V, iv-V. Half cadence can not be the last because it feels unfinished.
Plagal cadence is the progression of the subdominant chord and the tonic chord: IV-I, ii-I, iv-i, ii-i. Plagal cadence closes a phrase but not the whole composition.
Deceptive cadence is the progression of the V chord to the VI/vi chord. It feels interrupted and incomplete. It is the weakest cadence of all.