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Probably no musical genre has played such an important role in modern musicas as the blues. One can say that the blues is basically the founder of all forms of modern music. In fact, if we look at a sort of genealogical tree, it would without a doubt be at the base.
Historically the blues was born in the second half of the 1800's, when music was added to the part that for a long time was only vocal. In fact the primary form of blues was in the songs sung by black slaves in the plantations where they were forced to work for the "white master". Songs that were for the most part related to their unfortunately sad social situation. Some of the characteristic themes in the texts were of a sexual nature or dealt with the differences between whites and blacks, even if today it is clearly less. The expansion of rural blues (therefore limited to certain areas), to what would be the diffusion on a worldwide basis, would obviously bring some changes in the contents of the text, such as more modern and updated texts that pertained to the actual matters of the time in which a song was written.
The rhythmic division within the blues is fundamental, mostly due to the shuffle time, obtained by playing the octaves as if they were tercets. Ahead we will see how to use the rhythmic shuffle.
Melodically these songs could be identified in an African pentatonic scale. These scales evolved through harmonic and melodic elaboration, thanks to the initial use of musical instruments like the guitar, violin and harmonica. As the blues became more popular, any instrument could be used.
This introduction doesn't end here. To whomever is interested, we will take a closer look at the numerous texts on the market today.

The structure of the blues

Initially the typical structure of the blues was concentrated into a verse, followed by a rest (or an instrumental) that was repeated twice until the final resolution. Standard blues is defined as twelve bar blues, with some variations that we will see shortly. The twelve bars make up the chorus: a blues song is a series of repeating chorus'. Some are sung or instrumental, others are devoted to the improvisation, performing solos of one or more instruments.
In this section we will analyze the typical harmony of the blues and the many variations to its structure, in order to create a harmonic base sufficient to analyze the melodic construction. It's important to have a theoretical background, relavent to the construction of the scales, the chords and the use of the intervals. In fact, from this point on, we will use both technical and theoretical musical terms.

Twelve bar Blues

The typical twelve bar structure of blues is characterized mainly by the position of the chords, that can be put into three groups:
Tonic chords: chords built on the 1st position of the key;
Sub Dominant chords, chords built on the 4th position;
Dominant chords, chords built on the 5th position.
We will therefore have a structural composition where the chords are positioned in a precise form, as we can see in the panel below.

Adapting everything to a key, in C major for example, we can evaluate the relationship between the chords more precisely. We will therefore have these three chords:
C, Tonic, built on the 1st position;
F, Sub Dominant, built on the 4th position;
G, Dominant, built on the 5th position.
A very important detail is in the harmonic characteristic of a blues chord that, in a majority of the cases is the seventh chord. Whatever chord is found on the 1st, 4th or 5th, is always called the seventh: C7, F7 and G7. All of this has a fundamental influence on the melody and harmony of the blues. We will analyze this in more detail in the improvisation of the blues.
Lets therefore look at how the blues is structured in C.

The most common variation, that is also used mainly as the basic form, requires the use of degrees in a richer harmonic context, putting, for instance, the sub dominant in the second bar and varying the chorus in other areas, as you can see in the underlying scheme. This is the structure of a classical twelve bar blues.

You can listen to the base of a blues chorus by downloading the first or second midi file example.
The possible variations within a blues chorus are endless, and partly depend on the genre where you want to use it, such as jazz or rock. In the next page we will analyze some typical variations of the blues.