Harmonic progression
The order in which a piece's chords appear. Here you learn how a progression leaves the tonic and comes back to it, the commonest progressions in Western music, the direction the functions usually take and when music goes against it, root motion by fifths, thirds and seconds, the Picardy third, and what makes a chord connection smooth or angular.
In a song, the chords of the accompaniment change from time to time. Which chords appear, and in what order, is one of the things that most gives a piece its character. That order, from the first chord to the last, is called the harmonic progression (Blatter 2007, part 3, art. 6).
A composer may choose any chord at any moment. But some paths turn up far more than others, and knowing them is what lets you hear a piece and recognise, under the melody, a shape you have heard a hundred times.
Leaving home and coming back
Traditional music tends to begin on the tonic and, even more often, to end on it. The last chord of a piece is almost always the tonic, and almost always preceded by the dominant (Blatter 2007, part 3, art. 6).
The shortest progression that does this is I, V, I. It takes the music from home, the tonic, to a foreign place, the dominant, and back, completing a small journey (Blatter 2007, part 3, art. 6):
Swap the dominant for the subdominant and the journey changes colour, not shape:
The commonest journeys
Between the first chord and the last there is room for many others. Some progressions appear in so much music that they have become models (Blatter 2007, part 3, art. 6).
Tonic, subdominant, dominant, tonic:
The same, with the second degree between subdominant and dominant, and a dominant seventh:
Tonic, sixth degree, subdominant, dominant, tonic. It is the basis of many easy songs to play and sing (Blatter 2007, part 3, art. 6), and anyone who has strummed a guitar with friends has played it:
The direction of the functions
Looked at by function, these progressions share a direction: they leave the rest of the tonic, pass through the subdominant, reach the tension of the dominant and return to the tonic. Tonic, subdominant, dominant, tonic. The sixth degree, a relative of the tonic, extends the rest before the journey starts; the second degree, a relative of the subdominant, reinforces the move away.
That is the commonest direction in classical music, which is why dominant to subdominant sounds, there, like a step backwards. But it is no law. The blues, and much of the popular music that grew from it, takes exactly that step back, from dominant to subdominant, and makes it a signature (Blatter 2007, part 3, art. 6):
The twelve-bar blues
In the blues, the progression comes with its durations. One of the best-known models spreads the same chords over twelve bars, one chord to a bar, in three four-bar phrases (Blatter 2007, part 3, art. 6):
Four bars of tonic, two of subdominant, two of tonic, one of dominant, one of subdominant, two of tonic. Blatter draws attention to this: to sound like the blues, it is not enough to know which chords to play, you need to know when to play each one (Blatter 2007, part 3, art. 6). The rhythm at which chords change is harmonic rhythm, a subject of its own.
Major and minor
The same models work in minor keys, with the minor key's chords: I, V, I becomes i, V, i, and the progression with the sixth and second degrees becomes i, VI, iv, ii°, V, i (Blatter 2007, part 3, art. 6):
The dominant stays major, with the leading tone, because that is what pulls towards the tonic.
The Picardy third
A piece in a minor key sometimes ends on a major chord. The final tonic takes a major third, and the music, minor until then, closes in light.
The custom probably dates from around 1500, when the minor third was still thought too dissonant to serve as a final sonority, and it lasted until the end of the Baroque, around 1750. It is called the Picardy third, and it is still used, for its sound or to evoke older music (Blatter 2007, part 3, art. 6).
How the roots move
Another way to look at a progression is to follow the chords' roots. They move in three ways, and each sounds different.
By descending fifths. Each root is the dominant of the next. When that repeats over several chords, the progression follows the circle of fifths, and since any tonic can be preceded by its dominant, that order works in many styles (Blatter 2007, part 3, art. 6). It is the strongest motion, the one that most seems to move forward:
By thirds. Chords whose roots are a third apart share two notes, and the change is gentle, almost a change of colour. Going down by thirds, each chord changes a single note:
By seconds. Neighbouring chords have no note in common, and the change is sharp. It is what happens from subdominant to dominant, and from dominant to the sixth degree in a deceptive cadence.
Smooth or angular
Blatter sums up the question of common tones in two rules. The more notes two successive chords share, the smoother the harmonic motion; with no shared notes, the sense of change is greater (Blatter 2007, part 3, art. 6). Changing only the inversion, or the layout of the upper notes, is the smallest motion of all: the chord stays, and only its shape changes.
And neither rule acts alone. Changes of register, tone colour, dynamics or rhythm can soften an angular connection, or give force to a smooth one (Blatter 2007, part 3, art. 6).
What comes next
The progression says which chords come and in what order. What is missing is when: how often the chords change, and where the changes fall. That is harmonic rhythm.
References
- Blatter, Alfred. 2007. Revisiting music theory: a guide to the practice. New York: Routledge.Consulted: part 3, art. 6