The great thing about Blues is you don’t need to know a lot before you can start playing. In fact, you can start playing Blues progressions with just three chords. Generally, most blues chord progressions are based on the 12 Bar Blues formula.
A typical 12 Bar Blues progression would look like the pattern/progression below. Notice all the chords are 7th chords, also known as dominant 7th chords. 7th chords could be referred to as “Blues Chords” as they have that bluesy sound and are the most commonly used chord in blues music.
12 Bar Blues in the Key of A
A7/D7/A7/A7
D7/D7/A7/A7
E7/D7/A7/E7
Understanding 7th Chords: Dominant 7th chords are major chords with an added note. The added note being the flattened 7th note of the scale of the chords root note. So if you want to play a A7 chord, you take an A chord and add G - G being the flattened 7th note of the A major scale.
Understanding 12 Blues progressions: The key is this “1-4-5″ formula”. The numbers represent chords based on notes of the Major scale. If we take the key of A, the A major scale, then our notes would be the root A, the 4th tone or degree D and the 5th tone E.
A Major: A-B-C#-D-E-F#-G#-A
Thus, A, D and E are our 1-4-5 chords for 12 bar Blues in A . It’s Easy Right?
Enjoy 
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