Chord substitution


Audio: chord substitution (0:14)

Chord substitution
Figure: chord substitution

chord substitution plays substitute chords by a woodwind choir. The chord substitution figure, the score, shows seven different ways to substitute a chord.

Chord substitution is the process of replacing one chord by another.

Substitute chords are used in jazz and functional harmony. In functional harmony, a substitute chord has a similar function to the chord it replaces. In jazz harmony, a substitute chord can be nonfunctional, in which case it produces a similar sound to the chord it replaces, similarity being judged by the number of notes the two chords share in common.

The simplest case of substitution is voicing. Any chord can be substituted by voicing the notes differently. The notes are the same, it is the voicing that changes and makes the substitute chord sound different. Whereas there are only 6 ways to voice a triad, and half of them are inversions, there are 24 ways to voice a seventh chord and only 4 of them are inversions. Bar 1 of chord substitution shows four different voicings of a C major seventh chord.

Using an enharmonic chord is another form of chord substitution. An enharmonic chord contains the same notes as another chord but is spelled differently. The classic case of enharmonic chords are the major sixth chord and its relative minor seventh chord. C6 and Am7, for example, are enharmonic chords. They both contain the same notes, ACEG, but the two chords are spelled differently. Bar 2 shows two more enharmonic chords, Cmaj7 and Em6, that share the same notes, BCEG. Enharmonic chords occur in functional harmony as well as jazz harmony but their use differs: in functional harmony, enharmonic chords have different functions, in jazz harmony, they have equivalent sounds.

Another simple way to write substitution is to contract an existing chord. A seventh chord can always be contracted to a triad. Am7 can be contracted to Am or C, as shown in bar 3, Cmaj7 can be contracted to C or Em, G7 can be contracted to G or Bdim. There are even more ways to contract an extended chord, as bar 3 demonstrates by contracting Em9 to Bdim. The quality of the substitute chord may change through contraction.

Chord extension, the opposite of chord contraction, is another technique for creating a substitute chord. Diatonic extension creates a substitute chord that contains more notes than the one it replaces. For example, Cmaj7 can substitute for C, Cmaj9 can sub for Cmaj7, and so on, as shown in bar 4. The extended chord always retain the same quality as the original.

Extending then contracting a chord is a very effective substitution technique. Am7 can be extended to Am9 then contracted to Cmaj7. Cmaj7 can be extended to Cmaj9 then contracted to Em7. This is a very common form of diatonic substitution that is used between two chords whose roots are a third apart. Cmaj7 and Am7 are substitutes, as are Cmaj7 and Em7. In these types of substitution the chord quality changes and it has a greater impact on tonality. Substitution by extension then contraction forms the basis for substituting a secondary triad for a primary triad in functional harmony. iii and vi, two minor triads, sub for the major I triad, ii subs for IV, and viio subs for V. It is also a useful technique for varying a chord progression. In functional harmony, for example, V-I, a perfect cadence, can be substituted by V-vi, a deceptive cadence. In jazz harmony the progression ii-V-I can be altered by substituting the first and last chords to get IV-V-vi, root progression by second, a world away from the original root progression by fifth. Bar 5 shows a minor sixth chord contracted to a triad, then extended to a minor seventh chord and then contracted to a nontertian suspended chord.

Inversion, enharmonicity, contraction and extension are simple and effective techniques for creating diatonic substitute chords.

Chromatic substitution opens up a whole new world of possibilities. Chromatic substitution involves the use of a note or notes that are not part of the prevailing key. The effect is to weaken, temporarily or permanently, the sense of tonality. Take, for example, substituting Cminmaj7 for Cmaj7. Only one note differs so the result is a slight change in the sound, but that note is a chromatic note and has a significant impact on the sense of key. There is a fine distinction to be made between calling this key change an example of chord substitution, and calling it parallel key modulation. A similar process occurs by substituting minor chords with major chords, for example, a I-vi-ii-V can be substituted to produce a I-VI7-II7-V Ragtime progression. This type of progression can also be considered as a chain of secondary dominants.

Tritone substitution is a form of chromatic substitution that is popular in jazz harmony, and is used in functional harmony too. It is based on the fact that a tritone is the only interval that remains the same after inversion. It works as follows. Taking G7, GBDF, as an example, the chord is contracted to the tritone BF, inverted to FB, and a new root added a major third below, Db, to make the chord DbFB. Respelling it as DbFCb, its enharmonic equivalent, makes it an incomplete Db7. G7 has been substituted by Db7 using tritone substitution. Another way to look at tritone substitution is that the root of the substitute chord is always a tritone above or below the root of the original chord. Tritone substitution is used in jazz harmony to substitute a dominant seventh, V7, with bII7 (or #I7, its enharmonic equivalent). The progression ii-V-I is substituted by ii-bII-I which produces a chromatic motion down by semitone in the root. Tritone substitution really pushes the boundaries of tonality. When the chords are incomplete, there are two notes shared in common and two that differ, when the chords are complete, as in bar 6, there are still only two notes in common whereas there are four notes that differ.

A diminished seventh chord is an excellent example of chromatic substitution using an enharmonic chord. Every major and minor scale contains a diminished triad formed from consecutive minor thirds. Extending a diminished triad by adding another minor third above or below creates a diminished seventh chord. The diminished seventh chord can act as a chromatic substitute. For example, Bdim can be extended to G#dim7 by adding a minor third below. Rearranging the notes in G#dim7 results in four substitute enharmonic chords: G#dim7, Bdim7, Ddim7 and Fdim7. Contracting any of these diminished sevenths produces four different diminished triads. This technique is used to write the chromatic progression Bdim-Fdim7-Ddim7-F# in bar 7. In this simple way, the key modulates from C to its most distant key, F#7.

A chromatic substitute chord always impacts on tonality. When the substitution is short-term, tonicisation, the effect is of a temporary change in key. When there is a series of chromatic substitute chords then tonality weakens considerably and chromatic substitution becomes indistinguishable from modulation.