In Lesson Two you were introduced to the 12 Bar Blues progression. 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.