The musical alphabet uses only seven letter names. A, B, C, D, E, F, and G are the only pitches you'll find. After you get to G, the cycle repeats, starting again at A. This is vital to understanding music theory.
The blues follows a strict regimen of chords. If you play the blues in G, you will play a G, C, and D chord. It will never change; these are the fundamental chords in that key. If you want to move or transpose the chords into a different key, knowing the names of the chords in the G key doesn't really help you very much. Music theory can help you turn a chord into something that can be played in every key. How? With numbers. Why numbers? Because numbers refer to every key, while pitches and note names are specific to one key. So how will these numbers help us? Let's look at the example of the twelve-bar blues in G again. This time, in addition to the chord symbols, you will see a corresponding number for each chord. Look at the example in FIGURE 2-12.
Music theory uses Roman numerals, rather than Arabic numbers. You'll notice that the first chord gets the number I. The chord that shares the same note as the key is always I. For example, in the key of G, a G chord is always I. We give it the number I because it's the first chord we deal with. The C chord has the number IV. Why IV? Well if you count the note G as I, how far up is the note C? (When you count letters in music theory, you always count the first note, don't skip it!) So G is I, A is II, B is III, and C is IV. That's why we call the C chord a IV chord. The root (name of the key) of that chord is four notes away from G. When you're in any key the I chord is considered the center; all the other chords are measured away from I. Using the same logic, the D chord is five away from G, so it is called a V chord.
Applying Theory to Your Playing
Let's turn this from music theory to music reality and apply the concept of numbered chords to playing those chords. These numbers make changing the key easy. Let's apply this to the twelve-bar blues. FIGURE 2-13 shows the same old trusty twelve-bar blues pattern, but instead of chord names, there are only numbers.
If you would like to play this in the key of A, simply play an A chord each time a I chord is indicated. The other two chords are IV and V. To find those, just count up the musical alphabet starting on A, and count up to get the chords. You should have found D as the IV chord and E as the V chord. Go ahead and plug them into the progression and play. Voilà! You have the blues in A. You can use either open chords or barre chords for the A, D, and E chords.
Does this work with every key? Yes, that's the beauty of music theory. It doesn't talk about specific chords, only the relationships between the chords. In the blues, the form and chord progressions are always the same. Using some theory, some brainpower, and five fingers on your hand to count with, you can now figure out how to play blues in any key. Try playing blues in the key of C using this same technique as practice. I is C, IV is F, and V is G. Try some other keys on your own.
The guitar is so well designed that it makes changing keys even easier than using theory. That's right, no counting or thinking involved! The guitar is unique this way—no other instrument works quite like it. Look at FIGURE 2-14, a twelve-bar blues progression with moveable barre chords on the low E and A strings.
Notice how the root of the G chord is the third fret, and the root of the C chord is the third fret of the string above? Lets not call the chords G and C, lets call them I and IV as theory tells us to. So if you have a I chord on the low E string, it doesn't matter what fret, the IV chord will be on the same fret just one string up. For example, in the key of A, if your I chord was on the fifth fret of the low E string, than the IV chord will be on the same fret, the fifth, just up one on the A string. Look at FIGURE 2-15.
This works in any key! No more counting, just move your chord up one string and you have an instant way to find the IV chord from the I chord.
We can do the same thing for V. The V chord is always two frets above the IV chord. So if the IV chord is on the fifth fret A string, then the V chord is on the seventh fret A string. This makes life so easy. You can simply apply this rule to the blues progression. Pick a key—any key—and start that chord on the low E string. To get a IV chord just move the chord up one string. The V chord is two frets above that. You can easily apply this to the previous blues shuffle pattern, FIGURE 2-11. This example uses a moveable chord shape. You can move it the same way you move the chords. Let's pick a random key to do this in. How about D? D is found on the ninth fret of the low E string. Place your moveable shape on the ninth fret, instant I chord. To get a IV chord, move the shape up one string, and stay on the same fret. To get a V chord, go up two frets from the IV chord to the eleventh fret and place your V chord there.
You can now figure out the locations for any I, IV, and V chord! As long as you follow the order set forth in the twelve-bar blues progression, you can now play blues in any key.
The I-IV-V progression is used in many songs, not just blues. You'll be amazed at how many songs you already know that use this progression.