The ii-V-I

In an effort to further consolidate information I've been gathering, I'm writing a series of posts on key topics relating to jazz harmony, where a short summary paragraph of the topic at hand will be written, simply so that I'm able to describe it.

For this post, I'll be posting about the ii-V-I progression, a vital part of many of the rules I've created for the ruleset.

The ii-V-I

The ii-V-I progression takes its name from the name of the chord relating to the tonic or root chord.  ii means the second chord, V means the fifth, and I means the root. This is based on the major scale the root key is derived from.

For example, the C major scale features the following notes: C D E F G A B C

By using every second note in this scale, we can begin to create chords:

C E G B - C major7 - I

D F A C- D minor7 - ii

E G B D- E minor7 - iii

F A C E - F major7 - IV

G B D F - G7 - V

A C E G - Am7 - vi

B D F A - Bhalfdim7 - VII

Roman numeral characters in lower case are minor chords.  

The ii-V-I in the key of C then would be Dm7 - G7 - Cmaj7, based on the C major scale.  There are various alterations and different versions of the ii-V-I (the minor ii-v) for example, which is taken from other scales) though for the purposes of this post we'll focus on the 2-5-1 in it's most basic form.

The ii-V-I is a very popular progression.  It is used widely in pop and jazz music - in jazz especially, the ii-V-I sequence is the most popular chord progression used (though in forms other than its simplest).

To sum up why this progression is so harmonically effective in a single post wouldn't do it much justice - entire essays could be dedicated to the discussion of this chord progression.  However, for the sake of this blog post and the portfolio, I'll sum it up very briefly.

The ii-V is effective due to the relation between the two chords guide tones.  A guide tone is simply the tone that guides the chords tonality.  In G7 for example, the guide tones are G (the root) B ( the third) and F (the seventh).  These are the guide tones because they guide the tonality of the chord - the G signifies the root, the third signifies the major tonality, and the seventh signifies the dominant seventh.  

Let's look at the notes of Dm7 and G7-

Dm7 - D F A C

G7 - G B D F

Let's now look at the guide tones of those two chords.

Dm7 - F C

G7 - B F

We can see that they share a common note - F.  In Dm7, the F is the third.  In G7 the F is the seventh. The C and B are also semitones apart - in order to reach the B of the G7 from the C of the Dm7, one simply has to drop the seventh by a semi-tone.  The seventh of the Dm7 drops to become the third and the third stays the same but becomes the seventh.  

Due to the simplicity of this change, a clear sense of progression can be heard when played.  

The V-I is effective as it invokes a perfect cadence.  A perfect cadence is 'perfect' because it sounds as if the chord has resolved completely.  Again, this could be explained os much more in depth, but for the purposes of this project and the blog, I've kept it simple.

Lets look at the guide tones in the V and I chord:

V - G7 - B F

I - Cmaj7 - E B

See something similar?  The B (the third of the G) is the same as the seventh of the C - just like the relationship with the ii and the V.  The F (seventh) of the G drops a semi tone to become the third of the C - again, just like the ii-V progression did.  ii-V and V-I are exact mirrors of each other, ending with a perfect cadence.  In the simplest terms possible, this is why the ii-V-I works so well.  The sense of progression and voice-led harmonic movement creates a chord sequence which is very pleasing to the those who's ears are used to westernised harmony, based on resolution.

While this was a brief overview, it shows how the ii-V-I progression has become so monumentally important whilst creating my reharmonisation ruleset.  Next we'll look at how to reharmonises the ii-V-I itself to create more colour within the reharmonisations.

For now, take a look at this YouTube video which shows the ii-V-I in use.