Diatonic chords: Definition, purpose, and application

Question

I'm struggling to understand how and why diatonic chords work and what their purpose is within played music.

To my understanding, diatonic chords are a triad on any degree, in any scale, where each triad note is 2 generic thirds apart.

If that's correct, it seems to fly in the face of how each scale defines chords. It seems like diatonic chords ruin the purpose of the established structure. I.E. why call it a major scale if we have a diminished chord in it. The diatonic chords can seem quite out of place.

• What is the purpose/relevance of diatonic chords in music?

• How/where they can be applied, I.E. what can be/has been done with them?

(Examples in written, visual, and/or audible fashion are very welcome. 😊)

Thanks!

[Edit] In review, the question was created out of a misunderstanding of what a generic 3rd was which gave the impression of diatonic triads being something different than your standard triad on a note from a diatonic scale.

Feeling very silly now understanding what a diatonic chord actually is...

Link to diatonic triads and generic 3rds intoduction page.

Thanks again for all the help on what was a pretty obvious topic! 😁

Answer 1

I'm not sure I understand what you think a diatonic chord is, but I suspect you have a misunderstanding.

Diatonic chords are simply the seven chords that consist solely of notes that come from whatever key you are playing in (assuming you're in a major or natural minor key, or one of the other four or five diatonic modes). In other words, there are no accidentals in the chord. The purpose of a diatonic chord is to harmonize some note in a scale with other notes from the same scale. A large number of simple melodies (and even many more complex ones) can be harmonized completely, or at least mostly, with diatonic chords.

There are seven notes in a diatonic scale, separated from each other by whole steps and half steps, in some permutation of the pattern WWHWWWH. Since there are never two half steps in a row, skipping over one note will always give us two whole steps (a major 3rd) or one whole and one half step (a minor 3rd), hence the term "generic third" to describe either. If you count all the pairs of adjacent thirds (there are seven) you will notice that there are three major chords (a major 3rd followed by a minor 3rd) and three minor chords (a minor 3rd followed by a major 3rd). In addition, there is the one odd exception where there are two minor 3rds in a row, resulting in a diminished chord. To illustrate, I'll show the pattern you get starting on each of the first few notes in the major scale:

• Starting on 1: (WW)(HW) = M3, m3 = major chord = I
• Starting on 2: (WH)(WW) = m3, M3 = minor chord = ii
• Starting on 3: (HW)(WW) = m3, M3 = minor chord = iii
• Starting on 4: (WW)(WH) = M3, m3 = major chord = IV
• and so on...

Because all diatonic scales have the same pattern (albeit rotated), all major and minor scales (and other modes) have this same basic inventory of 3 major chords, 3 minor chords, and one diminished chord. What differs is where these chords fall in the scale. In major scales, the chord that falls on the first note of the scale (called the tonic) will be major. In minor scales, the tonic chord will, of course, be minor.

Answer 2

xor7ommy,

Your question seems to be about determining the tonality of music. Is it determined by the diatonic chords? Is it determined by the scale/key being used?

I think the thing to consider is this: if the music is truly in a major key - let's assume C major - then the tonic chord C major should have some special importance. Usually that importance is achieved by starting and ending the music with the tonic chord and ending many phrases on the tonic chord.

If the tonic chord was not emphasized in this way, I think you would get the result you suggest. The music would not seem to be in a major key. So again, using the key of C major, if we emphasized the d minor chord in our phrasing, the music could start sounding like it was not actually in C major, but rather in D dorian.

So, I don't think tonality is defined simply by which diatonic chords are being used, but which ones are being emphasized.

A few asides:

Composers like Satie, Debussy, and Ravel played around with the diatonic series emphasizing different sounds, not quite major or minor, but sort of modal.

A related term is "Pandiatonicism."

A fun experiment is to play only major chords over the roots of a major scale. So, in C major you would be playing chords: C major, D major, E major, F major, etc. This will definitely not sound like a major key. This sort of demonstrates that a major key is not comprised of only major chords.

Interesting topic!

Mike