A chord progression is a sequence of harmonies. Just as dissonant harmonies seem to want to resolve to consonant ones, chords often move in predictable patterns as well. The tonic pitch sounds like home in a scale or in a melody, and the tonic chord (I) sounds like the central and most restful harmony. Thus, the most common goal of a chord progression is to approach I. Once the tonic chord is achieved, that can be the end of a piece, or any chord can follow I to start another progression.
Common Patterns in Chord Progressions
The dominant chord (V) often preceeds I in chord progressions because it seems to have the strongest drive towards I. The large-scale motion from I to V and V to I creates the basis for a great deal of music in the western world. On a small scale, however, there are other chords that can have dominant function and can be used instead of V to create variety. The most common substitution for V is vii°.
There are two chords that commonly preceed V (or vii°): ii and IV. These are called predominants. In jazz ii is more common, but IV is more common in rock.
One very common chord progression in popular music is the 12-bar blues. The harmonic pattern is quite simple: I-IV-I-V-I. To fill 12 measures (or bars) of music with these chords, the first I is repeated for the first four bars, then each of the remaining chords is used for two bars each:
This pattern may seem predictable and repetitive, but these features make the 12-bar blues a good framework for improvised solos, which are an important part of both jazz and rock. The blues genre and this blues progression were major influences in the development of both jazz and early rock 'n' roll. Here are some examples of songs that use the 12-bar blues: