Cadences, and how much each one concludes
The formulas that end a musical phrase, from full stop to comma, and why each one sounds as conclusive as it does.
A spoken sentence ends with punctuation, and the punctuation says how finished it is: full stop, comma, question mark. A musical phrase ends with a cadence, and there are only a few of them, each with a known degree of conclusion. Recognising them is hearing where the music breathes.
Authentic: the full stop
Dominant to tonic. It is the cadence that concludes, because it resolves the dominant's tritone onto the root and the third of the tonic.
It is more conclusive when both chords are in root position and the melody ends on the tonic, and less when one of them is inverted or the melody stops on the third or the fifth. Tradition calls the first a perfect authentic cadence and the second an imperfect one; in practice, the difference is between an ending that closes and one that leaves the door ajar.
Plagal: the amen
Subdominant to tonic. There is no tritone to resolve, so the arrival is gentler, almost an assent.
It is the "amen" cadence at the end of a hymn, and it is the one that comes after the authentic, as a second and calmer closing: dominant, tonic, subdominant, tonic.
Half cadence: the comma
Anything to the dominant, stopping there. The phrase is left hanging: it reached the tension and did not resolve it.
It is the cadence at the end of a question. The next phrase usually starts again on the tonic and ends with the authentic, and the two together form the question-and-answer pair that organises half the melodies there are.
Deceptive: the surprise
The dominant prepares the tonic and the music delivers something else, almost always the sixth degree.
The sixth degree shares two notes with the tonic, so the resolution sounds almost right, and the "almost" is the effect. The phrase has not ended; it has gained another stretch, and the real authentic cadence comes later.
The bass and the melody decide the weight
Two cadences with the same chords can conclude in different ways. What changes the weight is what sits at the edges.
In the bass, root position closes more than an inversion:
In the melody, ending on the root closes more than ending on the third or the fifth. A melody that finishes on E over a C major chord is home, but has not sat down.
Hearing the cadence in a song
Take a song and sing the bass of the last two chords of each phrase. If it rises a fourth or falls a fifth to the tonic, it is authentic. If it rises a fifth or falls a fourth, it is plagal. If it stops on the fifth degree, it is a half cadence. If the dominant arrives and the next chord is not the tonic, it is deceptive. Four questions cover nearly everything popular music does at the end of a phrase.
What to take from here
A cadence is punctuation. The authentic is the full stop, the plagal is the assent after it, the half cadence is the comma and the deceptive is the phrase that gains another piece. The weight of each comes from the function of the chords and from what the bass and the melody do at the edges. The II–V–I is the authentic cadence with a preparation chord in front of it, and that is where the reading continues.