This commonly-used 12 bar Blues variation substitutes a ii7-V7 progression for the V7 chord in bars 9 and 10…
… which adds more harmonic motion and interest and is the beginning of a jazzier sound…
Play along and sing the roots of each chord out loud (Hint: C=Do, D=Re, F=Fa, G=So). Doing so is an extremely effective and efficient way to do ear training.
Frank playing a real-life example based on the form above…