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Diatonic Harmony are the chords that are built using the notes of a major or minor scale. It is useful to know, since it is super easy to use these chords to for example add chords to a melody in major or minor.
Here is how it works:
On each note in a major or minor scale we can build three-note chords, or triads by adding two more notes (on top of the "scale-note"); each an interval of a third apart:
These chords are named Diatonic, since they are using the notes through [Dia-] the scale.
Chord progressions using notes not belonging in that scale, are instead called Chromatic Harmony.
Even though there are 7 different triads in the scale, there are actually only three types of triads, both in the major and the minor scale;
These three types of triads each have a specific sound; major is often called “happy”, minor “sad” and diminished is… well “mysterious” :)
But as you will see, the triads when used together in a chord sequence, also create friction between them as they progress.
Some will give a calm feeling while others will create tension. This is what is really interesting about the study of diatonic harmony!
Let’s look at where the triads are positioned in a major scale first. Here is a C major scale:
(The numbers under the notes simply show the scale degrees or the order of the notes in the scale.)
From each of the notes in the C major scale you can build triads, as you saw above. For these triads we use Roman numerals instead of regular numbers:
Analyzing these triads, we see that on the:
How are the triads used?
Diatonic harmony with its 7 different chords is great to use to harmonize any melody in major (or minor), but the most useful are the triads on the 1st, 4th and 5th step. They are also called:
In a major scale these main triads will be major, and in a minor scale they will be minor.
What about the other 4 triads?
When adding chords to a melody, each of the 3 major triads may also be swapped (for a nice variation) with a parallel minor triad 3 semitones below.
They are called:
The last triad, the diminished triad on the 7th step, though it sounds ‘weird’, is great to stack together with (or even without) the Dominant.
This creates a Dominant 7th chord (four note chord) and is like loading an already pushy chord with Red Bull: It becomes super-pushy and wants to go home to the Tonic, desperately!
How about other major scales?
The triads “functions” will always stay in the exact same order in any other major scale, even if the name of the chord changes.
A natural minor scale will also have the three types of triads (major, minor and diminished) as in the major scale, but in a different order. Here is a natural C minor scale with triads:
Now you can see that:
The most important triads for harmonizing a melody in minor are the minor chords. They are still called the Tonic, Sub-Dominant and Dominant; but since they are in minor they are now written with lowercase roman numerals (i, iv, v).
Strengthening the Dominant:
Since the Dominant (v) in minor is weak and doesn’t pull back home to the Tonic as strongly as a major triad*, it is common to raise the third in the Dominant triad in minor.
This turns it into a major triad, and the scale has now become a Harmonic minor scale. (And yes, this is why it is called Harmonic minor, since the 7th step has been raised to accommodate for the Dominant (Harmony or Chord) to become major!)
*The reason for this is that the interval between the 7th and the 8th step (the root) in a natural minor scale is a whole step. This does not lead “home” so much as in a major scale, where the interval between the 7th and 8th step is a half step. That 7th step tone is also called leading tone.
So, usually you will se the Dominant chord in minor being major (V) instead, as well as having an added 7th for extra effect! (V7)
All these names are called Functional, since they describe the diatonic function or role each chord has in the group. When you do a functional analysis of a composition, you try to find out exactly this.
Most people today use the Roman Numerals to describe these functions, But it is still important to know the “real” names to understand what they represent as a diatonic harmony.