The 12 Bar Blues in the key of G contains the chords G7 (I chord), C7 (IV chord) and D7 (V) chord. Below is the 12 Bar Blues starting on G7.
12 Bar Blues in G (Jazz Variation)
The Jazz variation of the 12 Bar Blues in G adds some extra chords. The 12 Bar Blues structure remains the same, but certain chords are substituted. For example, the G7 chord in the fourth bar is substituted with a Dm7 moving to an G7 chord.