The Reason it's 12 Bar Blues is because it is played over 12 bars. So let me show you the structure of the 12 Bar Blues.
You may notice that the last bar on the chart (turnaround bar) has two different chords in it like the second bar does. The chord shown on top of the bar (the V chord) is the chord you play if you are going to repeat the 12 bar blues back to the beginning. You use the I chord if you are ending the song.
When you play the 12 Bar Blues, you can't just go out there and play one chord per bar for 4 beats. You have to create a rhythm, shuffle, swing, or whatever brings the blues out.
The sound files below are a traditional 12 bar blues in the key of G.
|12 Bar Blues Sounds||MP3|
A great way to remember the IV and V chords is to study the Circle of Fifths.
For Example: If you are in the key of A, your IV chord would be D, and your V chord would be E. Notice that the D is one position counter clockwise to the A, and the E is one position clockwise of the A.
You can use any Dominant chord when you play the 12 Bar Blues. In other words, you can use Dominant 7th, 9th, or 13th chords. Earlier, I concentrated mainly on the Dominant 7th chords, but any Dominant chord will work just fine in the 12 bar blues.