12 Bar Blues is a pattern of chords which repeats every 12 bars. There are hundreds of well known songs based on this chord progression, i.e., they contain basically the same chords in the same order. 12 Bar Blues is commonly used in Rock music and is the basis of most Blues music. Some popular Rock and Roll songs which use this 12 bar chord pattern are:
Here is a 12 Bar Blues in the key of G major. This pattern of chords will probably sound familiar to you.
Here is another 12 Bar Blues, this time in the key of C. Notice the use of the Bbnote.