In my lesson Blues for Beginners I talk about the basic 12-Bar chord progression that I think every musician should know like the back of their hand.
In order to understand how the chords are numbered, and why they’re numbered as they are, please check out my Chords for Beginners lesson, if you haven’t done so already.
As I show in Blues for Beginners, the 12-bar blues consists of 3 phrases in an A A’ B format. Each phrase is 4 bars long. There are many versions of the 12-Bar Blues, but the most simplified one is like this:
First phrase: 4 bars of I chord
Second phrase: 2 bars of IV chord followed by 2 bars of I chord
Third phrase: 1 Bar of V chord, 1 Bar of IV chord, and 2 bars of I chord
A lot of beginning harmonica students get confused why the numbers of the chords don’t match the numbers on the harmonica.
When we are playing the harmonica in 2nd position on a C harp, we are playing in the key of G. If you don’t understand positions, check out the first few minutes of my video, Keys for Beginners.) In 2nd position, the -2 is our root note, our home base, our tonic, and so the I chord is built off of that note, and it turns out -2 -3 -4 make up our basic I chord:
-2 Root
-3 Major Third
-4 Perfect Fifth
-5 Flat 7th
In the Blues, all of our chords are Seventh chords, which are technically called “Dominant Seventh” chords which means they are major triads (Root + Major 3rd + Perfect 5th) with a minor 7th (also called a flat 7th) on top.
All this to say, our I chord for the Blues is -2-3-4-5. Also, as it turns out, the -1 is the same note as the -4, just down an octave, so, for our I chord we can draw anywhere between holes 1 and 5.
When we are playing in the key of G, the IV Chord is C (count up 4 notes starting with G=1, A=2, B=3, C=4.) On a C harmonica , any note you blow is in the C chord. So now we have:
I chord: -1-2-3-4-5 (any or all of these notes will sound good)
IV chord: 12345678910 (any or all of these notes will sound good, but you’d have to have a pretty big mouth to play 10 holes at the same time! lol.)
Finally, the V chord does not exist, so we can just play the root note, which in our example would be the note name D (G=1, A=2, B=3, C=4, D=5.)
So to review:
I chord: anywhere between -1 and -5
IV chord: blow anywhere
V chord: just the -1 or -4.
Ok, that’s enough nerdiness for 1 post! LMK if you have any questions!
Aloha,
Luke
and memorize!! 
You need to get to the point that you don’t even have to think about things he has written – they should become as natural to you as breathing! 
