We know all the chords of the major and minor key now. Time to talk about authentic chord changes and plagal ones. This will be of great influence on the choices you are going to make while creating your own harmony. Everybody knows that major is supposed to sound cheerful and gay, while minor sounds sad. Now it is time to introduce you to another dualism that lies hidden in the diatonic system which has much influence on the way a chord progression sounds; it is the difference between authentic and plagal changes.
We already know some authentic changes, like V – I and also IV – V. These are strong changes, the most convincing and confident of all. Authentic chord progressions are active, powerful and dynamic and have a straight forward effect. They are the hallmarks of the classical era. If you want your music to feel strong and confident you should use an authentic chord progression. The word ‘authentic’ means real and credible. This could give you the false impression that plagal changes are no good and should be avoided at all cost. In the 18th century the classical composers believed just that. They made use almost exclusively of authentic changes. They abandoned the old style and the plagal church-modes, and some composers even wrote that finally real music was made.
Plagal changes are static, weak and have a more subtle effect. Tension is released, they are relaxed, and sound like a sort of anticlimax. They were much in use in medieval and Renaissance music, and are often associated with church music. Nowadays they return in gospel-, blues- and also in pop-music. If you are aiming for a more gloomily, maybe even dark harmony, you should use the plagal changes. Luckily for us the music of the 19th and 20th century has given us back all the beautiful riches of medieval and Renaissance music, so that now we can make use of all possibilities of tonality, authentic and plagal.
In the diatonic system there are only six kind of chord changes possible, because as long as chords are concerned a second up is the same as a seventh down, a third up is the same as a sixth down, and a fourth up is the same as a fifth down. Three of these six changes are authentic, and three are plagal, and they are mirror images of each other.
Authentic chord changes
Plagal chord changes
Again a symmetric picture appears:
– A second up is authentic, while a second down is plagal.
– A third up is plagal, while a third down is authentic.
– A fourth up is authentic, while a fourth down is plagal.
Now we understand better why I – IV – V – I is called the complete authentic chord progression; all changes are authentic. And if we play it backwards we get the complete plagal chord progression I – V – IV – I, in which all changes are plagal. Play both progressions and listen to the specific quality of them!