Watch the They Might Just Start with the V Chord. Listen For It! online guitar lesson by Jeff Scheetz from Blues Jam Survival Guide

We don't always start the blues on the 1 chord. Many times we will start on the 5 chord. Basically starting from measure 9 of our 12 bar blues. Then go to the 4 chord, and then your turnaround. At a jam someone will usually just say "start on the five", right before it gets counted off, so be prepared for this!