The 12 bar blues structure (phrase 1, bars 1-4)

This is the first in a series where I will analyse the 12 bar blues form and demonstrate how it can be transformed from a simple 3 chord progression to a more harmonically advanced structure. Expanding the form will introduce some chord substitution principles, as well as chord progressions that occur throughout the jazz repertoire.

The 12 bar structure is comprised of three 4 bar phrases.  This basic version has just three chords,  G7 (I7),  C7 (IV7), and D7 (V7). The blues form generally uses dominant 7 type chords. This breaks the rules of music harmony because in the key of G the I chord would normally be a major7 type chord. In blues however, this conversion to dominant chords is standard practice.

The first four bars (phrase one) have only one chord, G7.

 This is followed by two bars of C7 returning to G7 for bars 7 & 8 (phrase 2). The last four bars each have a different chord D7, C7, G7, and D7 (phrase 3).

The dominant chord is often extended to include the 9th and the 13th. The chord chart above shows two positions for the 7 chord, and one position for the 9th and 13th chord.

The basic form can be altered in many ways as long as we keep the important structural points at bars 5 and 9. The first alteration in phrase 1 happens in bar 2. This is used in rock and blues styles as well as jazz blues and is known as a quick change, which simply means that we replace the G7 in the second bar with C7.

A further alteration can be made to the second bar by adding a diminished chord on beats 3 and 4, giving us this progression in bars 1-4.


Because of the symmetrical nature of the diminished chord, which repeats every three frets, it is an extremely useful device for chord substitution and improvisation. The C#dim chord contains the notes C# E G A#. These notes are all a minor third from each other (three frets). This means that taking any of the notes as the root gives us the same chord.  C#dim (or Db dim) is also E dimG dim, and A#dim (or Bb dim).  We can therefore substitute any of these four chords for the other in our progression.

The chord chart below has three different shapes for the dim7 chord. The next alteration in our form comes in bar 4. This introduces us to another common chord substitution principle. Whenever we see a dominant type chord we can insert another chord into the progression. This will be a minor chord whose root is a 5th above the root of the dominant 7th  chord. In this case our 7th chord is G7, and the chord we will add to the progression is D minor 7.

This chord progression, where the dominant chord is preceded by the minor chord a fifth away, is probably the most common chord progression in jazz and is known as a ii-V.

We can also alter the G7 chord at this point. The tension that this creates will be resolved when we land on the C7 on the first beat of bar 5. Altering the G7 means that we will flatten or raise the 5th, or the 9th. Here are a few examples of G7 with altered tensions.

Here is a list of possibilities for our first four bars. Remember, the diminished and minor chords are optional.

These are just some of the options available for phrase one. A future post will introduce the tritone substitution,  another important chord substitution concept used in jazz.

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