The standard 12-bar blues is a I-IV-V chord progression most typically divided into three four-bar segments. Blues progressions are almost exclusively played in 4/4 time and dominated by the root (I Chord), with the IV and V chords providing that extra bit of flavor to keep things interesting.
In whatever key you are in, 12-bar blues uses the same basic sequence of I, IV, and V chords. It is most easily thought of as three 4-bar sections. the first 4, the middle 4, and the last 4 bars. The first 4 bars just use the I chord - I, I, I, I. The middle 4 bars go IV, IV, I, I.
