This is where we could launch into a lot of theory about I, IV, and V chords. But before we try to describe the Twelve Bar Blues, lets hear what we're talking about.
At the end of of this post is a list of songs that use the Twelve Bar Blues structure. Here's one off that list, "Black Rat," by Big Mama Thornton, backed by the Muddy Waters Blues Band, (James Cotton, Harmonica; Otis Spann, Piano; Sammy Lawhorn, Guitar; Luther Johnson, Bass; Francis Clay, Drums). Play the song, listen closely to the first two verses, then read on.
Twelve Bar Blues songs like this one are written in many different keys, but they all use the same pattern of chords, exactly twelve (12) measures long in total. This pattern repeats until the song ends. The Twelve Bar Blues is made up of three sections, each four bars long. (If you're not familiar with using roman numerals to write chord changes, pretend we are in the key of C. The correct chords for this key will appear in parentheses)
1. In the first section we rest on our I chord, the chord that the key is named after. The I chord (C) is is our home chord - the song feels at rest here. (In some songs, we'll sneak over to the IV chord (F) in the second bar, but for the most part, we're hanging out on the I chord (C) during the first 4 bars).
2. In section two, we venture away from home to the IV chord (F) for the first two bars, then back to the home chord.
3. In section three we move all the way out to the V chord (G), then back to the IV chord (F) and finally, back home to the I chord (C). At the end of section 3 we do a "turnaround" to prepare us to repeat the form, which usually involves the V Chord (G).
This 3 part structure is also reflected in many 12 bar blues lyrics and melodies. Often, each section has its own line of lyrics. The first two lines are often the same (lyrically and melodically), while the third line completes the idea in the first two lines.
"You is one black rat, someday I'll find your trail
When you're playing a Twelve Bar Blues, use the lyrics and the melody to help you keep track of where you are in the structure. Listen to the singer and sing the melody and lyrics to yourself in your head, or out loud, while playing.
Eric is a musician, audio engineer, educator, and radio nerd based in Seattle, WA.