Music Theory: Introducing Diatonic Harmony

Diatonic is the term we use to describe music that uses only the pitches of a particular major or minor scale. Every note in the following excerpt is from the G major scale, therefore it is diatonic:

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If accidentals occur, that is, pitches that don’t belong to the indicated key, the harmony is chromatic.  Let’s look at the first part of the Haydn excerpt again, this time with the harmony. The bass of the next to last chord is a C-sharp. Because C-sharp is outside of the key of G major, that chord is a chromatic harmony:

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Scale Degree Names

In the music theory lesson on key signatures, we learned that tonality is a hierarchy of pitches where one pitch has prominence over the others. To clarify the relationships of the scale degrees, each is given a name. Tonic is the key note (), a fifth above is the dominant, a fifth below the subdominant. The mediant is midway between tonic and dominant; the submediant is midway between tonic and subdominant. The supertonic and leading tone are a step away on either side of the tonic (click on the image to enlarge):

Diatonic Triads in Major Keys

Every major scale has its corresponding triads. To create these triads, stack two thirds on each of the scale degrees:

The example above shows the diatonic triads from two different major keys, C and E-flat. The pattern of major, minor, and diminished chords is exactly the same for every major key: tonic is always a major chord, supertonic is always minor, and so on. Every major key has three major triads, three minor triads, and one diminished triad.

It is traditional to use roman numerals to designate the chords: upper case for major, lower case for minor, and lower case with a degree symbol for diminished:

The advantage of this system is that we can understand chord relationships regardless of key. For example, we can discuss a chord progression such as I-IV-V-I, and it will be the same in any major key.

At this point, you should take a few minutes and memorize the chord qualities for the major keys:

• Major triads: I, IV, V
• Minor triads: ii, iii, vi
• Diminished triad: vii°

Diatonic Triads in Minor Keys

The minor key triads are a little trickier because we have to deal with the different scale forms (natural, harmonic, and melodic). Which of the three scale forms do you think is the most common when building chords in minor keys? If you guessed harmonic, you’re right. All of the chords listed below are derived from the harmonic minor scale form, except for III, which is from the natural minor. III+ (an augmented triad) is derived from the harmonic minor, but it is not as common as III (a major triad), so it is made smaller. Just remember, when you build triads in any minor key, use the harmonic minor scale form, meaning you raise whenever it occurs as part of a triad, except for III.

Even though we use an accidental () when building chords in minor keys, we still consider the chords diatonic. Notice also that we use lower case letters (in this case “a:” and “c:”) to indicate minor keys:

As with the major keys, the best thing to do is to memorize the chord qualities that occur diatonically in minor keys:

• Major triads: III, V, VI
• Minor triads: i, iv
• Diminished triads: ii°, vii°
• Augmented triad: occasionally III+

Composers traditionally raised in the minor key, to restore a half step between and , strengthening the  leading tone’s tendency to resolve to the tonic. Listen to the two short chord progressions below. The first does not raise the leading tone of the dominant chord (creating a minor dominant chord). There is nothing wrong with the progression. And because of its novel sound, some might even prefer it. But the second progression, which raises the 3rd () of the dominant chord, has more tension because of the strengthened tendency of the raised leading tone to resolve to tonic. This tension is preferred in most music:

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Diatonic Seventh Chords in Major Keys

Just as we constructed triads on each scale degree of a major scale, we do the same for seventh chords. All we do is stack another third (a 7th above the root) on top of each triad:

Now, memorize the qualities of the seventh chords in a major key:

• Major seventh: I⁷, IV⁷
• Major-minor seventh (dominant seventh): V⁷
• Minor seventh: ii⁷, iii⁷, vi⁷
• Half-diminished seventh: vii⁷

Notice that there is only one major-minor seventh chord, and it occurs on the dominant scale degree. That’s why we often refer to the major-minor seventh as a “dominant seventh.”

Diatonic Seventh Chords in Minor Keys

As with the triads in minor keys, use the harmonic minor scale to build seventh chords, except for the tonic (i⁷) and the mediant (III⁷), in which case, use the natural minor. Just always remember to raise , of the dominant chord (the third of the chord) and , which is the root, of the leading tone chord:

Seventh chord qualities in minor keys:

• Major seventh: III⁷, VI⁷
• Major-minor seventh: V⁷
• Minor seventh: i⁷, iv⁷
• Half-diminished seventh: ii⁷
• Diminished seventh: vii°⁷

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