A progression of harmonic chords can be found in almost every musical composition. A chord is the sound of two or more musical notes combined. Musical theorists have had a good understanding of why and how chords change from one to the next for centuries. Chord substitution is the musical technique of not playing the next chord in a given composition and instead playing a different one that still adheres to the harmony principles. A good substitution is always derived in some way from the original chord that was intended to be played.
The course of music is determined its “key,” which begins with the harmonic backbone of a chord based on the key’s first tone. It’s known as the tonic chord. The tonic chord in C Major is made up of the three notes C, E, and G. Music follows a path from this tonic chord to its dominant chord, based on the fifth tone, despite the fact that this is a generalization. The dominant chords in the key of C are G, B, and D.
Music returns to the tonic chord after reaching the musical climax of the dominant chord. The chord progression is the creative, roundabout harmonic steps that music takes to go from its tonic chord to its dominant, and to a lesser extent back to tonic. Roman numerals have long been used music theorists to express these chords: I for the root, V for the dominant, and everything in between through VII. I-I-I-I / IV-IV-IV-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-V-
Any of these chords can be replaced with a different one. The song’s essential structure will be preserved if this is done while maintaining the harmonic connection between the preceding and succeeding chords. In the case of the blues, replacing the first tonic bars with a harmonic sub-dominant chord based on the key’s fourth tone — I-IV-I-IV — does not significantly alter the song’s sound, but it does add complexity.
A chord substitution can be classified into several different types. It’s possible to add another note. The seventh tone, for example, C, E, G, and B for the I7 or C-major-seventh chord, adds a tense, anticipatory sound to the original chord. It’s also possible to subtract notes from the original. A default change to the tonic chord might be the simplest chord substitution.
Chord substitutions are used both novice and professional musicians. Beginner students of an instrument may be given familiar music in which the original chords have been replaced with simpler chords that are more appropriate for the student’s skill level. The technique of chord substitution, on the other hand, is extremely difficult at a high level of instrumental skill, such as that of an improvisational jazz pianist.
The harmonic mapping of each note in a new chord within the established progression is the basic principle underlying the technique. Treating any chord as if it were the tonic and then playing its equivalent harmonic dominant instead is one of the most common substitutions, known as a secondary dominant. Playing the chord in its relative minor key, usually with the addition of the key’s sixth tone, is another option. For the melancholy sound of vi7 or A-minor-seventh, the I-chord in C-Major can be played as C-E-G-A.
Other, even more difficult chord substitution options exist. A new chord can be inserted as an intermediate step or bridge between two perfectly good harmonic chords in a progression, usually slightly discordant to the ear. Similarly, adding a second tone to the chord, discord can be introduced. The difficulty of using a “mu chord” stems from the need to resolve the dissonant sound with the next chord in the progression. Highly skilled musicians, such as improvisational jazz saxophonist John Coltrane, can substitute not just one chord, but multiple chords in succession.