Sometimes you come across a concept on the guitar that provides that “aha” moment that takes your understanding of the guitar fretboard to a whole new level. Learning to apply the guitar number system to chord progressions is one of those concepts.
The guitar number system is a way to refer to chords by using a number instead of by name. Chords are built from each degree of a scale, with the degree of the scale being assigned a number.
Playing by number makes it super easy to transpose chord progressions to a different key. You don’t necessarily have to know each chord by name if you understand how to recognize the relations of chords by number on the fretboard.
It’s also helpful when it comes to applying chord function to chord progressions.
Let’s start out with how the chords of a given key are numbered and then take a look at how these numbered chords can be applied to the fretboard.
Guitar Number System
Chords for a key are built from the scale for that key. If you’re not familiar with building chords from scales, you should first read Building Chords from the Major Scale to have a better understanding.
For this lesson, we’re going to use the key of A major. In the table below, you see all of the scale degrees and chords that are built from the A major scale.
|Key of A|
The numerals on top represent the scale degree from which each chord is derived. These are the chord numbers.
Note that this also denotes the chord quality. Uppercase numerals represent major chords while lowercase numerals represent minor chords. The 7 chord is diminished, which is represented with °.
These chord qualities hold true for all major scales. The one chord will always be major, the two chord minor, and so on.
Using the chord table above, if someone says play a I-IV-V chord progression in the key of A the chords would be:
- I = A major
- IV = D major
- V = E major
A chord progression of I – vi – iii – IV would be:
- I = A
- vi = F#m
- iii = C#m
- IV = D
From these examples you can see that the chord number is based on the scale degree from which the chord is built while the quality of the chord is noted by using either uppercase or lowercase numerals.
You can apply this to any key by simply changing the root chord, or one chord. If you want to move to the key of C major, you would use C major as your root chord. Likewise if you wanted to transpose to G major, etc.
This concept is pretty straightforward. So how do we apply this effectively to the guitar fretboard?
Applying the Chord Number System to the Fretboard
There’s a really powerful aspect of learning the guitar chord number system that can expand your understanding of the guitar fretboard immensely. I try not to overhype any given concept for learning the guitar, but this is one that can literally kick down the door to unlocking the fretboard.
To help simplify the concept, we’re going to lean on the CAGED system to help map the chords to familiar chord shapes.
Revisiting the CAGED System
If you’re familiar with the CAGED guitar system, you know that every chord can be mapped back to 5 basic major and minor chord shapes:
It’s important to know the CAGED chord shapes thoroughly. It makes it a lot easier to identify chord positions and the relation between the chords in that position.
Using the CAGED Shapes to Form Chord Positions
The CAGED system shows us that all of the chord shapes are connected to one another. The C shape chord connects to the A shape, the A shape connects to the G shape and so on.
If we use each chord shape to define a position, we end up with 5 different positions, just as we do with scales. For each chord shape we also have a related scale shape. Using the E form A chord (root on the 5th fret of the 6th string), we can see that the scale shape formed here is position 1 of the A major scale.
Since chords are built from scales, we can also form chords in the key of A major from the notes of the scale.
Let’s take a look at an example of the A major chord using the E form major chord shape.
E Form Position
At the top of the diagram you have the full major scale, highlighting the root, 2nd, 3rd, 4th, 5th, and 6th scale degrees. Each scale degree represents the root note for the chord at that degree.
Note: The 7th degree is left out intentionally as it’s a chord that just isn’t used very much.
From the diagram above we have the following chord forms:
|Key of A|
|CAGED Form||E Form||Gm Form||Am Form||A Form||C Form||Dm Form|
If we use this information to map out each chord, we can create a relationship between the one chord and every other chord in the key.
Why is this important?
Whenever the one chord is the E shape CAGED chord, the five chord in the same position will always be the C form chord.
Just like with scale patterns, the relationship between chord positions remains the same regardless of key.
So, if you’re playing a I – IV – V progression in the key of G, the chords will remain in the same position relative to each other if you move to the key of C, or D, or any other key.
Chord Mapping for the Remaining CAGED Shapes
Let’s now take a look at the remaining CAGED positions and learn the relative chord shapes for each.
D Form Position
In the first example above we used the E form chord, so let’s look at the next chord form, the D form. The D form is derived from position 2 of the major scale.
When the one chord is in the D form CAGED position, the other diatonic chords are as follows:
|Key of A|
|CAGED Form||D Form||Em Form||Gm Form||G Form||A Form||Cm Form|
C Form Position
After the D form CAGED shape, the shapes repeat starting back with the C shape. The C form chord is derived from position 3 of the major scale.
The chord forms derived from the C shape position are:
|Key of A|
|CAGED Form||C Form||Dm Form||Em Form||E Form||G Form||Am Form|
A Form Position
For the A form position, we’re going to put our root chord (A major) in its natural position as an open chord. Because of this, we need to work with the forms a bit due to the fact that we don’t have enough room at the head of the neck to fit the scale notes to their normal positioning.
The scale shape for the A form position would come from position 4 of the major scale.
Below are the chord forms for the rest of the diatonic chords.
|Key of A|
|CAGED Form||A Form||Am Form||Cm Form||D Form||E Form||Em Form|
You may notice here that the iii chord is played as an inverted chord with the ♭3 in the root position. You could also play it with the root on the 4th fret of the 5th string. Played this way, the chord would apply to both this position and the next position, which is the G form position for the one chord.
G Form Position
The last position formed by the CAGED chord forms is the G form position. This position is based on the 5th position of the major scale.
The remaining chord forms for the G form position are:
|Key of A|
|CAGED Form||G Form||Am Form||Cm Form||C Form||D Form||Em Form|
Mapping Minor Keys
The examples here will apply to any major key. For minor keys, the relative chord shapes remain the same, but we need to re-number the chords. Remember that for every major key there is a relative minor key which contains all of the same chords.
The relative minor key is found on the 6th degree of the major scale. For A major, the relative minor would be F#m.
To get the chord numbers for the relative minor key, you start with the 6th degree of the major key and renumber that as the one chord. The 7th degree of the major key then becomes the two chord, the 1st degree of the major key then becomes the 3rd, and so on.
|Re-numbering for Minor Keys|
It’s important to realize you’re using all of the same chords and that the related chord forms will all remain the same for each position. The only difference is how the chords are numbered.
Chord Progressions with the Guitar Number System
To take full advantage of the guitar number system you need to be able to apply it to chord progressions. This is the quickest way to commit it to memory and become fluent with transposing keys.
In the examples below, we’re going to take several chord progressions and apply the guitar number system to the chord positions formed by the CAGED system.
There are several different approaches you can take, but initially I prefer to go position by position so you’re not trying to learn too much at once.
However, instead of playing the chord progressions in a single key, I recommend to using the same position across several keys. Doing so reinforces the relationship between the chords of a given position rather than the position on the neck itself.
After you feel comfortable with each position, you can start taking the progression and utilizing it across several positions instead of keeping to just one.
You’ll quickly realize some chord positions work better together than others depending on the progression. This is a good opportunity to play around with major and minor triads to see if certain voicings work better than others.
Chord Progression Exercises
In the first couple of examples, I supply the chord diagrams to help guide you through the chord forms in the progression. After that, it’s good practice to try to figure out the chord forms on your own if you’re not able to instantly recognize the shapes.
You should practice these progressions in all positions and not limit yourself to just the ones shown in the examples below. As stated above, it’s also important to transpose the progression to different keys so as not to associate the chords to single location on the neck.
Feel free to experiment with the rhythm however you like. The important point to focus on is the learning the chord forms in each position.
Chord Progression 1 : I – IV – V
The first progression is a very common chord progression, I – IV – V. This progression is the basis of blues and used so much across many genres that it’s imperative to get this set of chords down.
Key of A Major, Position 1
In position 1, the root chord takes on the E form CAGED shape and is derived from the first position of the major scale.
Key of A Major, Position 3
Instead of moving up a position to position 2 of the major scale and using the D form chord, we skip to the 3rd position and use the C form root chord.
Key of G Major, Position 2
In this example, we switch to the key of G and use the D form root chord, or position 2 of the major scale.
More Chord Progressions to Play by Number
Play through the following chord progressions using the same concept as above. Move between all positions and keys. Remember, to change keys you just have to change the root chord from which you base the progression.
As before, you can use any rhythm you like so feel free to play around.
- I – V – vi – IV
- I – iii – vi – IV
- I – iii – ii – V
- I – V – IV – vi – V
You can also apply this concept to any songs or other chord progressions you already know.
The guitar number system is a really powerful concept to learn. Once you have it down, it really opens up the fretboard and improves your playing. You’ll find it not only helps you to better understand and write your own chord progressions, it’s really useful in a band setting when you need to play on the fly. So put plenty of time into mastering this one.