Twelve bar blues

Twelve bar blues is a typical blues chord progression, taking twelve 4/4 bars to the verse.

A basic example of the progression would look like this, using T to indicate the tonic, S for the subdominant, and D for the dominant, and representing one chord per measure:

T T T T
S S T T
D S T T

Many variations are possible. For instance, seventh chords are often used just before a change, and more changes can be added. A more complicated example might look like this, where "7" indicates a seventh chord:

T  S  T  T7
S  S7 T  T7
D  S  T  D7

When the last bar contains the dominant, that bar can be called a turnaround.

Finally, here is an example showing the pattern in the key of D, and how it fits with the lyrics of a given verse. One chord symbol is used per beat, with "-" representing the continuation of the previous chord:

D        -     -      -        
Woke up this morning with the 

G     -   -    -   D - - - D7 - - - 
blues down in my soul 

G        -     -      -        
Woke up this morning with the 

G7    -   -    -   D - - - D7 - - -
blues down in my soul            Saying "My

A     -        -     A7
baby gone and left me, got a 

G    -    -     G7 D - - - D - A A7
heart as black as coal"