Watch the Keep Your Cool online guitar lesson by Jasper Mortier from 50 Blues Bass Licks You Must Know
Another variation on the 12 bar would be the so called 'long I' especially in the verse. The first I would be 8 bars instead of 4 often with a stop before you go to the IV stretching the whole form to 16 bars. Solo's normally are played 12 bar. For examples of the driving rocker style listen to Scuttle Buttin' or Love Struck by Stevie Ray Vaughan; My babe or You ain't nothing but fine by the Fabulous Thunderbirds with Jimmie Vaughan on guitar.