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Chord Patterns: Diatonic Chords

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by Conrad Albrecht

You learned about the tonic chord in our article Chord Patterns: Home and Away. Now let's learn about some more chords: the diatonic chords.

Most chords in most songs are diatonic chords; in fact, many songs use nothing but diatonic chords. So, what is a diatonic chord?

A diatonic chord is a chord which uses only notes from the "current scale".

Scales

(Before continuing, you should understand half-steps, whole-steps and the 12 chromatic tones; see Theory Basics: 12 Chromatic Tones.)

To review, these are the 12 chromatic tones:

A
A#/Bb
B
C
C#/Db
D
D#/Eb
E
F
F#/Gb
G
G#/Ab

A scale is just a collection of tones selected from these 12 chromatic tones. Usually, the tones in a scale are no more than 2 (or occasionally 3) half-steps apart; otherwise it doesn't "sound like a scale". Some scales are much more popular than others, and these popular scales have names.

For example, here are the tones in a Bb major scale (click the notes one at a time, from left to right, to hear the scale):

Bb      C       D      Eb      F       G       A      Bb

Notice that this scale, the Bb major scale, has 7 different tones; it's common practice to repeat the 1st tone (Bb in this case) again at the end to "finish off" the scale on its home tone.

Basic Diatonic Triads

So, say you're writing a song in the key of Bb major, using notes from the Bb major scale. Then the "current scale" is the Bb major scale, and diatonic chords are any chords which use only notes from the Bb major scale.

The most basic diatonic chords are 3-note chords (triads) made from alternating notes from the scale. So our 1st diatonic chord consists of the 1st, 3rd, and 5th notes of the Bb major scale:

Bb    C     D     Eb     F     G   A

These 3 notes (Bb, D, F) form the chord known as the Bb major triad (or just the "Bb chord").

The next diatonic triad from our Bb major scale is made from the 2nd, 4th, and 6th notes:

Bb     C     D    Eb    F     G     A

These 3 notes (C, Eb, G) form a Cm chord ("C minor triad").

Roman Numeral Chord Symbols

When we think about how a chord works in a certain key, the exact name of the chord (like "C minor") isn't most important. More important is to describe how the chord relates to the key, for example, "the minor triad built on the 2nd note of the key's scale". To show this chord function we use Roman numeral chord symbols. For example, in the key of Bb, if we use a Cm chord, the Cm chord's Roman numeral symbol is:

IIm

This IIm symbol shows that the chord is a minor triad built on the 2nd note of the key's scale.

Since every major scale has seven different tones, and we can build a basic triad starting on any of these tones, this means that there are seven basic diatonic triads in any major key. Here are their Roman symbols:

I    IIm    IIIm     IV    V     VIm    VII dim.

Note: I'm using the Roman chord-symbol system from Berklee College of Music. There is another "classical" system which is somewhat different; for example, it uses a ii symbol where Berklee uses IIm.

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