Chords have a mind of their own in the context of a song. In any given key, certain chords are much better at following the previous chord than others. The harmonic properties of chord progressions lead our ears by a leash. For example a song WANTS to end on the I chord. The song will feel incomplete or have a sense of ‘hanging’ if a song ends on any other chord.
That’s our ears telling us we want resolution. See, while your mind not understand all the theory behind it, your ears certainly do.
Here are some ground rules of what chords want to do:
- The I chord can lead to any chord. That’s the privilege of being the I chord, because it’s the same as the key (song in C = I is C).
- The ii chord leads to the IV,V,vi.
- The iii chord leads to the ii,IV,vi
- The IV chord leads to the I,iii,V,vii
- The V chord leads to the I
- The vi chord leads to the ii,IV,V,I
- The vii chord leads to the I,iii
A chord may be built upon any note of a musical scale, therefore a seven-note scale allows seven basic chords, each degree of the scale becoming the “root” or “tonic” of its own chord. A chord built upon the note A is an A chord: however, since any progression may be played in any key, the fundamentals of harmony are best grasped by numbering the chords according to the step of the scale they are built upon, upwards from the key-note. The structural meaning of a harmony depends exclusively upon the degree of the scale.
Any major scale gives three major triads that together include, and so can harmonize, every note of that scale. They are based on the first, fourth, and fifth scale degrees (the tonic, subdominant and dominant).
The same scale also provides three relative minor chords, one related to each of the three major chords. These are based upon the sixth, second and third degrees and stand in the same relationship to one another as do the three majors.
Apart from these six common chords there will be one step of the scale that gives a diminished chord.