Progressions and cadences

End a phrase on V and it sounds like a question; follow V with I and it sounds like a full stop.

Chords have jobs

In music built on major and minor keys, chords fall into three groups by what they do, called harmonic functions. Tonic (T) is home and sounds stable: I, or i in minor. Dominant (D) creates tension that wants to resolve to tonic: V and vii°. Predominant (PD) leads away from home toward the dominant: IV and ii are the strong ones, and vi and iii often come before them. Learning which group a chord belongs to tells you more than its name does.

Phrases in this style usually move through the groups in one direction: tonic, then predominant, then dominant, then back to tonic, written T-PD-D-T. Textbooks call this the phrase model. I-IV-V-I in C major is C, F, G, C, a phrase that leaves home, builds tension and returns. I-ii-V-I (C, Dm, G, C) does the same job with a different predominant. In this style a phrase almost never runs backwards, from a dominant chord to a predominant one.

Cadences: musical punctuation

A cadence is the chord pattern at the end of a phrase, and it works like punctuation. An authentic cadence is V to I and sounds finished. It is a perfect authentic cadence (PAC) when both chords are in root position and the tonic is the top note of the final chord; otherwise it is imperfect (IAC), a little less final. A half cadence ends on V, whatever comes before it, and sounds like a comma or a question mark: the phrase pauses on tension and waits for an answer.

Two more cadences are common. A plagal cadence is IV to I, often called the "amen" cadence because it is the sound of the Amen sung at the end of many hymns; it usually comes after an authentic cadence as an extra close. A deceptive cadence sets up an ending with V and then moves to vi instead of I, so G goes to Am in C major. The ear expects home and gets a near relative, which keeps the music going. Robert Hutchinson's open textbook points to the prelude to Act I of Wagner's opera Tristan und Isolde for one of the most famous deceptive cadences.

You can hear all of these on the guitar chords below. Play C, F, G, C for a phrase that ends with an authentic cadence. Stop on G and the phrase hangs in the air, a half cadence. Play G and then Am and you have a deceptive cadence. Finally add F to C after the last C and you hear the plagal close. The same moves, written as numbers, work in every major key, which is why chord charts in Roman numerals are so useful to working musicians.