12 Bar Blues

The 12 Bar Blues is simply a chord progression. It is a I - IV - V progression. This progression is often used in jam sessions. Most of the time, musicians use Dominant Seventh Chords for this progression. So for the key of G you would use G7, C7, and D7.
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.

Figure 1

Figure 2
Bar Number 1 2 3 4 5 6 7 8 9 10 11 12
Traditional 12 Bar Blues I7 I7 I7 I7 IV7 IV7 I7 I7 V7 IV7 I7 V7/I7
Modern 12 Bar Blues I7 IV7 I7 I7 IV7 IV7 I7 I7 V7 IV7 I7 V7/I7
Now, Notice that the first 4 bars are G7. That is the Traditional way to play the first 4 bars. You might want to try the Modern method by substituting the IV chord into the 2nd bar. (as shown on the chart) I would advise you not to try the Modern version until you know the Traditional version.
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.
Cycle of Keys
The circle as you should already know goes up a Perfect Fifth as you go clockwise around the circle. As you go counter clockwise it goes a Perfect Fourth. In other words the IV and V chord are to the left and right of the root chord.
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.