II-V-I

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ii-V-I is a very common chord progression used in a wide variety of music genres. It is a succession of chords whose roots descend in fifths from the supertonic to the dominant and finally to the tonic. In a major key, the supertonic (ii) triad is minor, while in a minor key, this triad is diminished. The dominant chord is, in its most basic form, a major triad. With the addition of chord alterations and extensions (most often sevenths), limitless variations exist on this simple formula.

[edit] Jazz

ii-V-I progressions are extremely common in jazz. They serve two primary functions, which are often intertwined: to temporarily imply passing tonalities, and to lead strongly toward a goal (the "I" chord). One potential situation where ii-V-I progressions can be put to use is in a blues, whose generic form has no such progressions. In this example, a simple 12-bar F blues is shown followed by a similar one with some basic ii-V-I substitutions:

| F7 | Bb7 | F7 | F7    | Bb7 | Bb7 | F7 | F7    | C7 | Bb7 | F7 | C7    |
| F7 | Bb7 | F7 | Cm F7 | Bb7 | Bb7 | F7 | Am D7 | Gm | C7  | F7 | Gm C7 |

In bar 4, instead of the simple V-I root motion in the original blues, the ii chord of the Bb7 (Cm) is included so that the measure is even more directed toward the following downbeat with the Bb7. In bars 8-10, instead of leading back to the tonic with the standard V-IV-I, a series of applied ii-V-I progressions is used to first lead to Gm, which then itself is reinterpreted as a ii and used to lead back to F7 through its own V, which is C7. In the last bar (the "turnaround"), the same type of substitution is used as that in bar 4. In practice, musicians will often add extensions to the basic chords shown here, especially 7ths, 9ths, and 13ths, as seen in this example:

IIm9 V#9b13 Imaj9

In jazz, the ii is typically played as a minor 7th chord, and the I is typically played as a major 7th chord (though it can also be played as a major 6th chord). The iim7-V7-Imaj7 progression provides smooth voice leading between the thirds and sevenths of these chords; the third of one chord becomes the seventh of the next chord, and the seventh of one chord moves down a half-step to become the third of the next chord. For example, in the key of C, the standard jazz ii-V-I progression is Dm7-G7-Cmaj7, and the thirds and sevenths of these chords are F-C, B-F, E-B; inverted for smoother voice leading, these become F-C, F-B, E-B.

The ii is sometimes replaced by the II7, giving it a more dissonant, bluesy feel; this is especially common in turnarounds. Additionally, the ii can be treated like a temporary minor tonic, and preceded by its own "ii-V", extending the basic progression to a iii-VI-ii-V-I; again, this is quite common in turnarounds (with the iii-VI replacing the I in the second-to-last bar; in the example above, the last two bars would change from F7 | Gm C7 to Am D7 | Gm C7).

The ii-V7-I can be further modified by applying a tritone substitution to the V7 chord, replacing it with the bII7 chord. This is possible because the bII7 has the same third and seventh as the V7, but inverted; for example, the third and seventh of G7 are B and F, while the third and seventh of Db7 are F and Cb, which is equivalent (enharmonic) to B. Performing this substitution (in this case, changing Dm7-G7-Cmaj7 to Dm7-Db7-Cmaj7) creates smooth chromatic movement in the chord roots--the root of the ii (D) moves down a half-step to become the root of the bII7 (Db), which moves down another half-step to become the root of the I (C).

The tritone substitution, the substitution of II7 for iim7, and the iii-VI-ii-V extension can be combined in different permutations to produce many different variations on the same basic progression--e.g. iiim7-bIII7-iim7-bII7-Imaj7, III7-bIII7-II7-bII7-I7, etc.

[edit] Classical

In the tonal tradition, the ii-V-I progression is most often reserved for cadences, and is one of many often used cadential progressions. The ii, V, and I can all appear in inversion, although usually without significant alteration beyond the addition of sevenths. One very common implementation of ii-V-I in a classical piece would be this progression, where the ii chord appears in first inversion:

ii6 V7 I

In minor, a seventh chord built on the supertonic yields a half-diminished seventh chord, which is a very strong predominant chord. Due to what is considered the harsh nature of root position diminished chords, the ii° chord most often appears in first inversion.