Music Theory for Harmonica Players Part 11 – Defining a 12 Bar Blues

In the last post we looked at the notes that make up the I7, IV7 and V7 chords in the key of G. Now it’s finally time to examine how these chords are used in the context of the most common type of blues.

The 12 Bar Blues

You’ll likely be familiar with the term 12 bar blues. You’re almost certainly used to hearing it, whether you realise it or not.

For our purposes, a bar is simply four beats. If you were tapping your foot along to the music your foot would tap four times for each bar.

So four beats equal one bar. A 12 bar blues is made up of (no surprises) 12 bars. A total of 48 beats.

Here’s how that might look in a diagram. Each box represents a bar, so we have three rows of four boxes to make our 12 bars.

Over the duration of these 12 bars the band is going to play the I, IV and V chords in a specific sequence. This is called a chorus. At the end, they’ll go back and do the whole thing over again. Then again, and again and again until the song finishes.

The most common 12 bar chorus structure looks like this.

For the first four bars the band stays on the I chord. For bars five and six they change to the V chord. Back to the I chord for bars seven and eight. Then we have one bar of the V chord (bar nine), one bar of the IV chord (bar ten), then back to the I chord again for bar eleven.

Let’s take a look at those last four bars. It will become useful as we progress to think of these as being divided into two sections. The V-IV-I transition and the turnaround.

We’re not going to go in depth at the moment but it’s useful to be aware of so store that knowledge away for now.

Let’s put this theory to use by doing some active listening. Here’s Jimmy Reed’s Bright Lights, Big City on YouTube.

After a four bar introduction the song settles into its 12 bar pattern. If you look at the diagram while counting along with the song you should begin to see how it all fits together. It’s really quite a beautiful thing.

I’d encourage you to actively listen to as many blues as you can and see if you can count along and anticipate the chord changes. You really want to get this into your bones. You may notice the drums often build up to the change, and the guitars may build in anticipation too.

After a while, you should be able to start a song at a random position and pinpoint where they are in the progression within a bar or two. Might sound impossible now if you’re new to this but trust me. Time and listening will get you there.

Good artists to listen to include Muddy Waters, Sonny Boy Williamson (both I and II), Howlin’ Wolf, Jimmy Rogers and of course Jimmy Reed. Not everything you come across will be a straight 12 bar, and some that are may have minor variations. We’ll cover those in a later post.

Meanwhile, in the next post, we’ll start playing along with the 12 bar blues ourselves.