Chords in the 12 bar blues sequence

The name 12 Bar Blues comes from the number of measures or bars in most blues songs - twelve.
It is, at its most basic, based on the I-IV-V chords of a key.

What are the I, IV, and V Chords?

The I, IV, and V chords are the three most used chords in each major key. Aloud you would call them, “The one, four, and five chords.”  The I chord is built on the first note of the key. The IV chord is built on the fourth note of the key. And, the V chord is built on the fifth note of the key.

For example, the key of C major is spelled C, D, E, F, G, A, and B. The first note is C, the fourth note is F, and the fifth is G. The I, IV, and V chords in the key of C are a C major triad, an F major triad, and a G major triad. We’d simply say, “The chords are C, F, and G.”

     I                                                          IV               V

       One                                                Four          Five

C	D	E	F	G	A	B	C
1	2	3	4	5	6	7	8

From the above table we can see that the 1st note in a C major scale is C, the 4th note of the scale is F, and the 5th note of the scale is G. Let's look at the selected 1, 4, and 5 in a different table:

C major chord	F major chord	G major chord
ONE of the scale	FOUR of the scale	FIVE of the scale

You should notice the notes of each triad fit within the key. Remember, that’s what diatonic means – within the key. C major is made up of the notes C, E, and G. F major is F, A, and C. And, G major is G, B, and D. The key is what binds the chords together into a small family.

I, IV, and V Chords in Each Key

Here is a table of the I, IV, and V chords in all 15 major keys. Don’t kill yourself trying to memorize these all in one shot. With time and experience you will eventually memorize these groups of chords. You're going to encounter them so much that you’ll come to see and hear them as a small family.

Major Key	I	IV	V
C	C	F	G
G	G	C	D
D	D	G	A
A	A	D	E
E	E	A	B
B	B	E	F#
Cb	Cb	Fb	Gb
F#	F#	B	C#
Gb	Gb	Cb	Db
C#	C#	F#	G#
Db	Db	Gb	Ab
Ab	Ab	Db	Eb
Eb	Eb	Ab	Bb
Bb	Bb	Eb	F
F	F	Bb	C

Scales and Keys:

Let's bring you up to speed on the basics.  Look at your piano keyboard.  Notice the black keys are in groups of two's and three's.  Find a group of two.  The white key to the left of the first black key in any group of two is a C.  Good  Now, notice that if you start with C and count up 12 keys, you'll be at C again.  There are really only 12 unique notes in Western musical notation.  Easy right?

Right.  Now, most every piece of music you hear will be in a "key".  This means that the composer decided that one of the 12 notes would be more important than the rest. For the purposes of this column, we're going to be in the key of C Major.  If you start with a C and play (up the keyboard) all the white keys until you get to the next C, you will hear what C Major sounds like.  It should sound very familiar to you.

Now, let's talk about ways to identify the notes in the C Major scale (and other major scales). C is the first note in the scale also called Scale Degree One, and also called Tonic.  Next we have D, scale degree 2, Supertonic.  E, scale degree 3, Mediant.  F, scale degree 4, Subdominant.  G, scale degree 5, Dominant.  A, scale degree 6, Sub-mediant.  B, scale degree 7, Leading Tone.  And then we're back to the C that's one octave above our original tonic.  Your first step in becoming a theory wiz, is to memorize the names of the notes in the major scale. (Note: These names apply to any major scale. Not just C Major.)  Here they are again:

C - Scale Degree 1 - Tonic
D - Scale Degree 2 - Supertonic
E - Scale Degree 3 - Mediant
F - Scale Degree 4 - Subdominant
G - Scale Degree 5 - Dominant
A - Scale Degree 6 - Sub-mediant
B - Scale Degree 7 - Leading Tone 

The 12-Bar Blues Form

The most common form of the blues is a 12-bar pattern of chord changes. That is, a repeated twelve-bar chord progression. This is called “12-Bar Blues”.
You should remember a bar is the same as a measure. Most often in blues you will count 4 beats to each bar – 4/4 time.
In a blues song this 12-bar cycle gets repeated over and over through the course of the tune. A blues song might play through it 20 times. It will depend on the specific song.
The blues can be played in any key. 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.
And the last 4 bars go V, IV, I, V. Then you repeat the whole thing again.
So, the basic 12-bar sequence looks like this: