Eight bar blues

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 beat:

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

(The same chord progression can also be called a sixteen-bar blues, if each symbol above is taken to be a half note in 2/2 or 4/4 time -- blues has not traditionally been associated with notation, so its form becomes a bit slippery when written down.)

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  T  S7 S7 
 T  T  T7 T7
 S  S  S7 S7
 T  T  T  T
 T  T  T7 T7
 S  S  S7 S7
 T  T  D7 D7
 T  T  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   -   -   -  
 blues down in my soul            

 D                -               D7         -
 Woke up this morning with the blues in my soul

 
             G              -     

 Saying "My baby gone and left me, got a 

 G7              - 
 heart black as coal" 

 D              -               
 Woke up this morning with the 

 A          A7       D      -   A7   -
 blues down in my soul