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.
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Information from- www.studybass.com
