Analysis
The twelve-bar blues, the form that is a harmony
Twelve bars, three chords, all of them dominants, and a form that has been repeating for a century; the analysis of a structure that belongs to nobody and that every popular musician plays.
The twelve-bar blues is a form, and at the same time it is a harmony: whoever says "a blues in C" has already said the chords, their order and how long each one lasts. It is the most stable structure in popular music, played all over the world without a score, and analysing it is analysing why three chords are enough.
Three chords, three phrases
The twelve bars fall into three phrases of four. The first stays on the tonic, with a visit to the fourth degree in the second bar in many versions. The second goes to the fourth degree for two bars and returns to the tonic for two. The third goes to the dominant, drops to the fourth degree and returns to the tonic: that is the cadence, stretched over four bars.
Read by degree: I, IV, I, I; IV, IV, I, I; V, IV, I, V. Tonic, subdominant and dominant, the three functions, and nothing else. The form is the phrase of tonal harmony repeated in fixed proportions.
All of them dominants
What makes the blues sound like the blues, rather than like a hymn cadence, is that all three chords have a minor seventh: C seventh, F seventh, G seventh. In functional harmony only the fifth degree would be a dominant; here the tonic and the subdominant are too, and so no chord really comes to rest. The tritone in the tonic chord, E and B flat, contradicts the C major scale, and that contradiction is what gives the genre its roughness.
Over those chords the C blues scale works from the first bar to the last: the scale's minor third, E flat, against the chord's major third, E, is the other half of the sound.
The harmonic rhythm and the turnaround
One chord per bar, always, which is what gives the form its predictability. The exception is the end: in the last two bars the return to the beginning asks for a turnaround, and the simplest version is the one the scheme shows, C and G seventh, the tonic and the dominant that hand the music back to the first bar. More elaborate versions put four chords into those two bars, the I, VI, II, V or its equivalents with dominants, and the harmonic rhythm accelerates exactly where the theory says it does: before the return.
The form as a contract
Twelve bars and three phrases are a contract between the players: any musician can join a blues in any key without rehearsing, because the form says everything. That is why the blues serves as the ground for rock, for jazz and for improvisation in general: it is not a song, it is a place where songs happen.
What the form teaches
That a form can be a harmony; that tonic, subdominant and dominant are enough for twelve bars; that the minor seventh on every chord is a choice of genre, not a mistake; and that the turnaround is the harmonic rhythm accelerating into the return. Everything the harmony courses teach fits into these twelve bars, and whoever knows them by heart knows half of it.