| 12 bar blues – Here’s how I think of it.
Let’s turn 12 bars of nuthin into a blues form.
|| I | I | I | I | I | I | I | I | I | I | I | I ||
Traditionally, the first four bars is like a call. Second four bars is a repetition of that call. Last four bars is an answer/response to the call, so let’s separate it into those sections.
|| I | I | I | I || I | I | I | I || I | I | I | I ||
The answer should have some finality, so let’s make a cadence.
|| I | I | I | I || I | I | I | I || V | V | I | I ||
Both calls are exactly the same. That’s boring. I usually say it more emphatically the second time, since they probably didn’t hear it the first time. We’ll give a musical reason to draw in the listener.
|| I | I | I | I || IV | IV | I | I || V | V | I | I ||
Hmm, the answer seems a little dismissive, don’t you think? Let’s make it address the question a little more relevantly.
|| I | I | I | I || IV | IV | I | I || V | IV | I | I ||
Okay, now to bluesify it.
|| I7 | I7 | I7 | I7 || IV7 | IV7 | I7 | I7 || V7 | IV7 | I7 | I7 ||
That’s a basic blues form. There are a lot of different modifications that are frequently made, though.
|| I7 | IV7 | I7 | I7 || IV7 | IV7 | I7 | VI7 || ii7 | V7 | I7 | I7 ||
and stuff.
“Rhythm changes” refer to the changes to “I’ve got Rhythm”, and adaptations/variations of those changes. They follow an AABA structure, with each section 8 bars long, going something like this:
A:
|| I vi7 | ii7 V7 | I vi7 | ii7 V7 | I I7 | IV ii7 | V7 | I V7 ||
A:
|| I vi7 | ii7 V7 | I vi7 | ii7 V7 | I I7 | IV IV#dim | V7 | I ||
Bridge:
|| III7 | III7 | VI7 | VI7 | II7 | II7 | V7 | V7 ||
A:
|| I vi7 | ii7 V7 | iii vi7 | ii7 V7 | I I7 | IV IV#dim | V7 | I ||
You could probably find faster and more complete answers to this stuff doing a google searches . | 
