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Students always have similar questions about the diatonic chords. This topic is so important, it's good to over-explain it and make sure you get it...

Why Are Some Chords Major and Others Minor?

On the last page, you probably noticed that some chords were major, while others were minor or diminished. Why, for instance, aren't they all major if they're in a major key?!

The diatonic chords of the major scale must stay within the notes of the key. Because of the way the notes are spread out in the scale, some chords will fall major and others minor (and one lonely diminished chord).

For instance, the D major chord contains the notes D, F# and A. The D minor chord has the notes D, F and A. If we're in the key of C major, we can't use an F# and remain diatonic. So the chord which fits the key of C would have to be D minor and not D major.

The same is true for the rest of the chords in the key—some are major (I, IV and V), some minor (ii, iii and vi) and one of them is diminished (vii).

The Same Seven Chords in All 12 Keys

Remember, since all major scales have the same pattern and structure, the pattern of diatonic chords is also the same in every key. Once you've learned one key, you know the diatonic chords in all major keys. It will then come down to knowing the notes of each key.

For example, the IV chord is always major. That means in the key of D, the IV chord is a G major triad. The 4th note in the key of D major is G (D, E, F#, G, A, B and C#).

The vi chord in every major key is always minor. In the key of E major (E, F#, G#, A, B, C# and D#) the 6th note is C#. The vi chord in the key of E major is C#m.

What's the Reason for Numbering the Chords?

When you understand the numbering system, it will simplify a lot of learning, playing and hearing.

If someone put song charts in front of you having these progressions:

C – G – Am – F

Gb – Db – Ebm – Cb

E – B – C#m – A

You might think you have 3 songs to learn, but they're really the same. The progression is I – V – vi – IV for all of them.

With more experience you will start to hear songs and progressions in terms of their position in the key. You'll think, “Oh! They walked down to the vi chord!” Or, “That's just a I – vi – ii – V turnaround.”

There are many common progressions using the diatonic chords. When you hear them, you'll more quickly be able to find them on the bass and know what you can play on them.

You can expect other musicians to use the numbers when giving you instructions such as:

“Vamp on the V chord.”
“Change that vi chord to a IV chord in the bridge.”

Another reason is for transposing (shifting from one key to another). If you understand the numbers of the chords in the progression, you can quickly and easily change a progression into any other key.

As you can see, there are lots of reasons to study the diatonic chords. Don't forget bass players are the foundation of the harmony. The diatonic chords are a big part of harmony.

Are There Non-Diatonic Chords in Songs?

Yes! We will learn more about these later, but there are a number of commonly used chords which fall outside of the key. These will confuse you while you're trying to analyze the diatonic chords in songs. These non-diatonic chords usually exist to connect from one diatonic chord to another.

A quick example of common progression in the key of C might be: C – C7 – F. The C7 chord contains the notes C, E, G and Bb. There is no Bb in the key of C. The C7 chord is then not diatonic, but it makes for a great transition to F (the IV chord).

Don't worry about these non-diatonic chords yet, just be prepared for some chords in songs not to fit perfectly into the seven main diatonic chords.

In Summary

Diatonic means from the notes of a key.

Each major key contains seven notes. Each of those seven notes can be the root note of a chord.

The chords are numbered using Roman numerals: I, ii, iii, IV, V, vi and vii.

The numbers make hearing, understanding and playing chord progressions easier.

In the next lesson coming up, we'll learn to play the diatonic chords in a very convienent way—all in one place on the fretboard!