The II-V-I Progression - Introduction
The II-V-I progression is the most common progression in jazz and popular music. It is a slight modification of the IV-V-I progression that is so common in classical music. You can find II-V-I progressions in classical music but they are not as common as the IV-V-I progression. In a major or minor key, the II and IV chords share tones in common and therefore sound very similar. It seems that, since jazz musicians liked progressions of a 5th so much, they popularized the II-V-I progression.
The II-V-I Progression - Cadences
In both classical music and jazz these progressions are very powerful in establishing a key. They often serve as cadences to finish a harmonic progression at the end of a phrase. The chords of the II-V-I progression are normally diatonic chords of the key. In a major key, the progression is IImi7 to V7 to Ima7 (In the key of C major - Dmi7 to G7 to Cma7). In a minor key, the progression is IImi7b5 to V7b9 to Imi#7 (in the key of C minor - Dmi7b5 to G7b9 to Cmi#7).
The II-V-I Progression - Bracketing
Since all of the chords of the II-V-I progression come from either a major scale or a harmonic minor scale, it is possible to bracket the progression with a single scale! Bracketing means treating a chord progression of two or more chords with a single scale sound. This both strengthens the sound of the key and simplifies improvisation over the progression!
The II-V-I Progression - Major key
All of the chords of the II-V-I progression in a major key come from the major scale of the key. Therefore, only the major scale of the key is needed to improvise over the progression. Of course, attention has to be paid to giving emphasis to important chord tones such as 3rds and 7ths! Technically the Dorian scale is used with the IImi7, the Mixolydian scale is used with the V7 and the Major scale is used with the Ima7. But they are all modes of the same major scale so an improvisor may simply think of the major scale of the key over all three chords!
The II-V-I Progression - Minor key
All of the chords of the II-V-I progression in a minor key come from the Harmonic Minor scale of the key. Therefore, only the Harmonic Minor scale of the key is needed to improvise over the progression. Of course, attention has to be paid to giving emphasis to important chord tones such as 3rds and 7ths! Technically the 2nd mode of the Harmonic Minor scale is used with the IImi7b5, the 5th mode of the Harmonic Minor scale is used with the V7 and the Harmonic Minor scale is used with the Imi#7. But they are all modes of the same Harmonic Minor scale so an improvisor may simply think of the Harmonic Minor scale of the key over all three chords!
The II-V-I Progression - Digital Patterns
Digital patterns are conceived in numbers (digits) relating to the position of the notes in the scale or the key. For example this pattern, 1, 2, 3, 4, 5, 6, 7, 8, 9, starts on the 1st note of the key (scale) and moves upwards stepwise through the scale, ending on the 9th which is an attractive target note on the I chord. This pattern, 4, 3, 2, 1, 7, 6, 5, 4, 3, starts on the 4th note of the scale and descends through the scale to the 3rd of the I chord, always a good note to emphasize!
By thinking numbers instead of specific notes, it is easy to reproduce patterns in any key! The other nice feature is that most patterns will sound equally good in either a major or a minor key! The Major scale of the key is used with the II-V-I in a major key and the Harmonic Minor scale of the key is used with the II-V-I in a minor key. II-V-I progressions may occur as a one measure phrase, a two measure phrase, a four measure phrase or even longer. Usually they are shorter phrases at a slow tempo and longer phrases at a fast tempo. Digital patterns may be condensed or extended to fit different phrase lengths.