The 12 Bar Blues in the key of C contains the chords C7 (I chord), F7 (IV chord) and G7 (V) chord. Below is the 12 Bar Blues starting on C7.
12 Bar Blues in C (Jazz Variation)
The Jazz variation of the 12 Bar Blues in C adds some extra chords. The 12 Bar Blues structure remains the same, but certain chords are substituted. For example, the C7 chord in the fourth bar is substituted with a Gm7 moving to an C7 chord.