12 Bar Blues progressions contain the first I, fourth IV and fifth V chords of a particular key.
The V chord is usually a seventh chord. Call the key chord I and count up to the fourth (IV) and fifth (V) chords. Here is a 12 Bar Blues with the chord numbers only. This 12 bar pattern remains the same and from it you can work out the chords for a Blues in any key.
To improvise against a Blues progression, use Pattern 1 in the position commencing at the same fret (on the 6th string) as the keynote.
Other common keys for playing 12 bar Blues are listed here, along with the correct chords for playing Blues in each of these keys. Eventually, it is worth learning to play in all keys.
Blues in the Key of E (Uses E, A and B7 chords)
Blues in the Key of D (Uses D, G and A7 chords)
Blues in the Key of C (Uses C, F and G7 chords)
Blues in the Key of A Minor (Uses Am, Dm and E7 chords)
Blues in the Key of E Minor (Uses Em, Am and B7 chords)