12 Bar Blues is a pattern of chords in a particular order which repeats every 12 bars. This progression is widely used in Country music. An example of this sequence is shown below.
Roman numerals are often used to replace chord symbols to indicate the basic chords used in 12 bar progressions. It is important for you to remember that 12 Bar blues in any key uses the 3 chords, I, IV and V in the following sequence.
For example in the key of C the I, IV and V chords are C, F and G respectively. In the key of G the I, IV and V chords are G, C and D.