The 12-bar blues is the most common form of the blues. It is the foundation for songs in many styles of music, so it will serve you to become familiar with playing it in all keys. Because most musicians are well versed in this musical form, it is usually the first common ground that people find in a jam session.
The chord progression is three lines or segments that are each four measures long. Here is an example of the progression, using chord numbers:
I / / / I IV / / / I I / / / I I7 / / / I
IV / / / I IV / / / I I / / / I I / / / I
V / / / I IV / / / I I / / / I (V7) / / / :II
You may play any or all or the chords as either major triads or dominant 7th chords, but you will probably want to play the last measure of the first and third lines as seventh chords, as marked. The last chord is in parenthesis, as it will only be played before repeating the entire sequence. When you want to end the song, simply finish with the I chord.
Here is the progression in the key of A:
A / / / I D / / / I A / / / I A7 / / / I
D / / / I / / / / I A / / / I / / / / I
E / / / I D / / / I A / / / I (E7) / / / :II
Here it is in the key of E:
E / / / I A / / / I E / / / I E7 / / / I
A / / / I / / / / I E / / / I / / / / I
B7 / / / I A / / / I E / / / I (B7) / / / :II
Although you will eventually want to memorize the progression in all 12 keys, first become familiar with it in the keys that are most easily (and therefore commonly) used on the guitar: C, A, G, E and D. (If you need help understanding keys, refer to A Guitar Player’s Guide to Music Theory or A Guitar Player’s Guide to Ear Training.)