Most all musical compositions can be structurally defined as a progression of harmonic chords. A chord is the combined sound of two or more musical notes. Several centuries of musical theorists have developed a good understanding of why and how chords change from one to the next. In a given composition, chord substitution is the musical technique of, not playing the next chord, and instead playing a different one that still adheres to the principles of harmony. A good substitution is always, in some regard, derived from the original chord meant to be played.
The course of music is established by its “key,” and starts from the harmonic backbone of a chord based on the first tone of the key. It is called the tonic chord. In the Key of C Major, the tonic chord consists of the three notes C, E and G. Although it is a generalization, music’s path is from this tonic chord to its dominant chord, based on the fifth tone. In the Key of C, the dominant chord is G, B and D.
From attaining the musical climax of the dominant chord, music then returns to the tonic chord. The creative, round-about harmonic steps music takes to go from its tonic chord to its dominant, and to a lesser degree back to tonic, is the composition’s chord progression. The traditional notation of music theorists to express these chords are Roman numerals — I for the root, V for the dominant, and everything between through VII. A 12-bar blues song might be transcribed: I-I-I-I / IV-IV-I-I / V-V-I-I.
Any of these chords can be substituted with another. If done so while retaining the harmonic connection between its preceding and succeeding chords, the song’s essential structure will remain. In the blues example, chord substitution in the first tonic bars with its harmonic sub-dominant chord based on the key’s fourth tone — I-IV-I-IV — will not markedly change the song, but give it a more complex sound.
Categorically, a chord substitution falls into several different types. Another note can be added. The addition of the seventh tone, for example — C, E, G an B for the I7 or C-major-seventh chord — gives the original chord a tense, anticipatory sound. Notes can also be subtracted from the original. The simplest chord substitution might be a default change to the tonic chord.
Chord substitutions are practiced by both amateur and proficient musicians alike. Beginner students of an instrument may be provided familiar music whose original score of chords have been substituted with simpler ones more appropriate to the student’s skill level. At a high level of instrumental skill however, say an improvisational jazz pianist, the technique of chord substitution is an extremely difficult one.
The basic principle underlying the technique is the harmonic mapping of each note in a new chord within the established progression. One of the more common substitutions, called a secondary dominant, is to treat any given chord as if it were the tonic and then to play its equivalent harmonic dominant instead. Another substitution is to play the chord in its relative minor key, usually with the addition of the key’s sixth tone. The I-chord in C-Major can instead be played as C-E-G-A for the melancholy sound of vi7 or A-minor-seventh.
There are other even more difficult options for chord substitution. A new chord, usually slightly discordant to the ear, can be inserted as an intermediate step or bridge between two perfectly good harmonic chords in a progression. Similarly, discord can be introduced by adding a second tone to the chord. Popularly called a “mu chord,” its difficult use comes from the necessity of resolving the dissonant sound with the next chord in the progression. Very skilled musicians such as the improvisational jazz saxophonist John Coltrane can substitute, not just one chord, but several successive chords.