Chord selection can be seen as providing context and direction to a melody.

Context provides a background against which the melody note will be heard. If the melody note is member of a the chord, the chord provides consonant support. If the note is not a member of the chord, the listener knows that the note is "less important", perhaps only a passing dissonance in the grand scheme of things.

Direction refers to how the harmony and melody unfolds over time. For example, stronger chord movements (movement of roots by fifths) may imply a stronger cadence, while a more linear progression (inversions where roots move stepwise) may imply a weaker and more local movement.

One way of measuring the dissonance of a chord is by the harmonic intervals. Using the overtone series as a guide, the strongest interval is the octave/unison, followed by the perfect fifth/fourth, and then the major third/sixth.

The perfect fifth is a very strong interval, second only to the octave/unison. In both the diminished and augmented chords, it is subverted. The diminished chord also has a minor third instead of a major third, but this is secondary to the diminished fifth.

If you think of chords as coming out of scales, the diminished chord comes "naturally" from the diatonic scale, starting on the 7th degree of the major scale.

C major scale = C - D - E - F - G - A - B - C

B dim chord = C - D - E - F - G - A - B - C - D - E - F - G - A - B - C

With no perfect fifth or major third, it's a very weak chord. Since it's built on a stack of minor thirds any the notes of the chord have equal claim to being the tonal root of the chord.

In this context, a dim7 is a stack of four minor thirds. Again, any of the four notes could make equal claim to be the tonal center.


In contrast, the augmented chord doesn't appear "naturally" in diatonic scales. While the diminished chord doesn't imply any harmonic direction (all the notes of the chord could equally be called the root), the augmented chord very much creates an interval which needs to be resolved.

Hopefully this information will help explain why you might choose to use these chords:

1. You might choose to use a diminished chord in a place where you need to provide consonant harmonic support to passing tone, but don't want to draw attention to it.

A typical example of usage here might be where you've got some horns on a part, and the melody has a passing note, such as C - D - E. The overall harmony in the example is C major, but (for the sake of discussion), let's say you want to put a consonant chord under the passing D note.

In theory, you could choose to harmonize the D note with something like a G7 chord:

C -> G7 -> C

But a G7 is harmonically strong, and would draw a lot of attention to that point. You could choose another diatonic option (chord drawn from the scale) such as:

C -> Dm -> C

...but even that might be too strong. On the other hand, if you chose a really weak chord (such as a diminished chord):

C -> Ddim -> C

Voila! You have harmonic support (so the horns have motion) without drawing too much attention to that particular note.


2. You might have a linear bass line over a held chord. For example, the progression:

Am -> Am/G# -> Am/G -> F#dim -> Fmaj7

can be seen as:

Am -> Am/G# -> Am/G -> Am/F# -> Am/F

The diminished chord arises over a held chord (Am) over a moving bass line.


3. In a jazz context, a diminished chord can be seen as dominant 7th chord without a root. For example, G7 = G+B+D+F. Bdim = B+D+F.

So G7 = Bdim/G


4. Unlike a diminished chord, the augmented chord calls a lot of attention to itself. It's typically used to add a chromatic movement to a progression, with the progression resolving by continuing the motion up another half-step. For example:

C = C+E+G -> Caug = C+E+G# -> Am = C+E+A

Here, using the Caug provides consonant support for the G - G# - A melodic movement.

Often, the chromatic movement may appear in the melody, but it may appear in the root of the chord instead. In the example above, it's just a chromatic embellishment to a simple C -> Am progression: interesting to hear, but just a passing motion in the grand scheme of things.


-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?