Harmonic substitution

In music theory, chord substitution is the technique of using a chord in place of another in a sequence of chords, or a chord progression. The vast majority of blues, jazz and rock music songs are based on chord progressions.

Given that in all of the named genres, and many others, a chord progression is repeated to form a song or tune, composers, songwriters and arrangers have developed a number of ways to add variety to a repeated chord progression. Of course, there are many ways to add variety to music, including changing the dynamics (loudness and softness) or instrumentation. This article is about the use of chord substitution to create variety in a chord progression.

Jazz musicians, both chord-playing sidewomen and men and composer/arrangers often substitute chords in the original progression to create variety and add interest to a piece. The substitute chord must have some harmonic quality and degree of function in common with the original chord, and often only differs by one or two notes. Scott DeVeaux describes a "penchant in modern jazz for harmonic substitution."

One simple type of chord substitution is to replace a given chord with a chord that has the same function. Thus in the simple chord progression I-ii-V-I, which in the key of C Major would be the chords C Major-d minor-G Major-C Major, a musician could replace the I chords with "tonic substitutes", the most widely used of which are iii and vi (in a Major key), which in this case would be the chords e minor and a minor. This simple chord progression with tonic substitutes could become iii-ii-V-vi or, with chord names, e minor-d minor-G Major-a minor. The musician typically uses her/his "ear" (sense of the musical style) to determine if the chord substitution works with the melody. There are also subdominant substitutes and dominant substitutes.

The ii-V substitution is when a chord or each chord in a progression is preceded by its supertonic (ii7) and dominant (V7), or simply its dominant. For example, a C major chord would be preceded by Dm7 and G7. Since secondary dominant chords are often inserted between the chords of a progression rather than replacing one, this may be considered as 'addition' rather than 'substitution'.

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