This commonly-used 12-bar Blues variation substitutes a full turnaround based on secondary dominants V7/II-V7/V-V7 in the last two bars…
Doing so creates lots of harmonic interest, more interesting bass lines, strong harmonic voice leading, and rhythmic momentum to the top of the next chorus…
Play along and sing the roots of each chord out loud (Hint: C=Do, D=Re, F=Fa, G=So, A=La). Doing so is an extremely effective and efficient way to do ear training.
Frank playing a real-life example based on the form above…