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Music Theory:
Chord Families and Diatonic Substitution


CHORD FAMILIES:

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Chords may be grouped into families based upon the principle chords found in harmony. These principle chords include the Root, Fourth and Fifth. (Notated as: I, IV, V).

In the key of C Major, we have the Root, (or the I chord), occurring as the C Major chord. The fourth step chord, (or the IV chord) occurs as the F Major. And, the fifth step chord, (or the V chord), presents as G Major.

The three main harmonic families:

chord_families

These families are named as the; Tonic, (I), Sub-Dominant (IV), and the Dominant (V). The notes of each family combined spell out every tone of the major scale. The movement from one chord to another creates the basic harmonic effects of all tonal music.

It is important to memorize the function of each chord as well as the other chords which relate to each chord family.

FUNCTIONS:
- Tonic Family
The function of the Tonic family is to temporarily, or permanently begin, or end a piece, or section of music.

- Sub Dominant Family
The function of the Sub-dominat family is to move-away from the tonic family and move toward the Dominant family.

- Dominant Family
The Dominant family wants to resolve back toward Tonic. The pull of the 3rd chord tone (the leading tone of the key - in the case of C Major it is a B note) combined with the root of this chord (in Cmajor it is a G note) being out by a fifth. And, the major second interval of the chords 5th chord tone, (in C major it is a D note) all work together by surrounding the arrival of the movement toward notes of the Tonic chord. In the end the result is a very strong resolution. In Classical theory it is referred to as an Authentic Cadence.


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DIATONIC CHORD SUBSTITUTION:
Diatonic Chord Substitution is an example of simple reharmonization using chords of the same key that share several notes with the three principle families. The chart below illustrates how the three chord families of the I, IV and V have cousins under them.

Cousins of the Tonic family include the III and the VI. Under the IV chord (the Sub-dominant family) the cousin is the II chord. Under the Dominant family (the family of the V chord) we find the VII chord.

Study the chart below:

diatonic substitutes

To more clearly show Diatonic Substitution in action, I have created two similar chord progressions in the key of C Major.

In Progression One we find harmony occurring as; I, VI, V, IV. In Progression Two I have swapped the Ami chord (the VI) for it's Diatonic Sub. the III (an Emi chord).

Play through each example to hear for yourself how easily the substitute chords can swap out for each other.

Progression One: C Major ( I, VI, V, IV)

progression_one

Progression Two: C Major ( I, III, V, II)

progression_two

Upon listening, it is easy to notice that although the progressions are indeed different, they do share a similar emotional effect.

Before I wrap up, it is also important to mention that chord voicings (arrangement and location of chords on the neck) also play a role in how similar the substitutions effect will come across to the listener.

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