Most blues songs follow a form called the 12-bar blues. In addition to being 12 bars in length, the 12-bar blues has its own internal logic. After you grasp that logic and use it to shape your phrases, playing the blues on your harmonica will seem as natural as talking. A verse of 12-bar blues has three parts, and each part is 4 bars long.
Each part does several things:
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It advances the narrative of the words.
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It builds on the previous part.
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It prepares the following part.
Some 12-bar blues just use one background chord that plays behind the entire tune. However, most blues tunes have a chord progression, or sequence of chords. Each part of the blues verse starts with a different chord, and that helps you know where you are in the verse. Musicians have come up with thousands of variations and sophisticated elaborations on the 12-bar blues chord progression. However, here I stick with the most basic, down-to-earth version. It uses just three chords, which you can identify by their relationships with one another, using roman numerals:
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The I (one) chord, which is also the home chord that's identified with the song's key
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The IV (four) chord, four steps above the I chord
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The V (five) chord, five scale steps above the I chord
The following figure shows 12-bar blues as a chord chart, which presents the chord progression of a tune and how long each chord lasts. Each diagonal slash represents one beat, with the vertical bar lines marking the end of each 4-beat bar. The chords in parentheses are optional. They don't occur in the very simplest version of the 12-bar blues, but players use them very often.
