Harmonizing the diatonic scale produces chords with different sounds. These chords have different qualities which produce different effects when played in a piece of music. Though these chords offer a wide range of expression, musicians often want to add variety and surprise to their music. This is accomplished by substituting existing chords with ones that sound different but accomplish the same harmonic function. When the new chord belongs to the same key as the original chord, it is called a diatonic substitution.
Just as scale degrees are designated by Arabic numerals (1, 2, 3...), chor ds are similarly designated by Roman numerals (I, II, III). For example, in the key of C major the notes are C,D,E,F,G,A,B and we have designated them as 1,2,3,4,5,6,7. However, when we consider the triad chords in the key of C we have Cmaj, Dm, Em, Fmaj, Gmaj, Am, Bdim. These are designated I,II,III,IV,V,VI,VII. When speaking about these chords, for instance in the key of C major, we would call Gmaj the five chord, Dm the two chord, Fmaj the four chord and so on.
There is no single agreed-upon system for numbering chords. One system uses upper-case and lower-case Roman numerals to distinguish whether a chord is a major or a minor (Dm = ii, Fmaj = IV). The Harmonizer uses upper-case Roman numerals for all chords. However, if the chord happens to be minor, a lower-case "m" will appear next to the Roman numeral (ex: VIm). In the case of diminished chords such as the seven chord, the Roman numeral will be followed by a superscript circle (ex: VII[). A four-note dominant chord will be written V7.
Major and Minor
In the Common Chord Substitution section of the Harmonizer the one chord (I) of the major scale is found at the top of the column of chord roots. This chord is derived from the major key as appearing in the key designator of the Major and Natural Minor Scales section. Since the natural minor scale begins counting from the sixth note of the major scale, the chord that has this note as its root is the one chord (Im) of the minor scale relative to the major. Though they share the same chords, they perform different harmonic functions.
Name and Function
This column indicates the root of the chord, its chord name, and the theoretical name which describes the chord's harmonic function. Each of the seven chord degrees (I, II, III, etc.) performs a slightly different function in a piece of music. Knowing a chord's function is important when making chord substitutions.
The root note in the window combines with the suffixes to the right to spell out the name of the chord which is to be replaced. The theoretical names as presented in this section apply only to the chords of the diatonic major scale.
A chord's theoretical name indicates its relationship to the other chords of the key and gives musicians a way to talk about chords without being specific to a key.
A theoretical name is similar to the name given to a position on a baseball team, like pitcher, catcher, or outfielder, etc. The theoretical name describes what function the chord will play in a piece of music. Some chords create tension, some relaxation, while some create movement. While it is important to know all the positions a chord can play, the most popular chords in pop music are the tonic, subdominant, and dominant. In the key of C major these would be C, F, and G, respectively.
For the purpose of substitution, it is important to understand what position a chord plays so the proper substitution can be made. Musicians also refer to chord positions by their Roman numeral equivalents. The tonic is the one chord. The subdominant is the four chord. The dominant is the five chord.
The chords in the windows to the right of the original chord represent frequently used substitutions. Substitutions do not work in every case. There are some specific and even complicated rules for substitution, but the final arbiter of a successful chord substitution is how it sounds. We will look at three basic rules for substitution --- chord family, inversion, and flat five.
Chord Family Substitutions
It is helpful to know the functions of the chords in order to make successful chord substitutions. The diatonic structure consists of three families of chords: tonic, subdominant, and dominant. The tonic family expresses the tonal foundation of a key. The subdominant family expresses movement away from the foundation. And finally, the dominant family expresses harmonic tension. This tension is released with chords that move the harmony back to the tonic. Chords belonging to the same family can often be substituted for each other.
The diagram below shows the similarity between notes when chord families in the key of C major are grouped together.
Another kind of chord substitution is accomplished by rearranging the notes of a chord to produce a new chord. Below is an example of some of the chords in the key of C major that are inversions of each other and can usually be used interchangeably.
Because of its construction, a diminished seventh chord (dim7) offers several inverted combinations that work as chord substitutes. This chord is made up of four notes spaced a b3rd (minor third) apart. The notes of the diminished seventh repeat every 1-1/2 steps up or down the scale. The diminished seventh chord is not diatonic to the major scale; instead it is derived from the harmonic minor scale.
Flat-Five Substitution (Tri-tone)
Another kind of chord substitution which is often used in pop and jazz music is know as flat-five substitution. This substitution is made by replacing a dominant chord with a new dominant chord whose root is a diminished fifth interval above the original chordÕs root. For example, in the key of C major, a Gdom7 (G7) could be replaced with a Dbdom7 (Db7). (G to Db is a diminished fifth interval.) This is not a diatonic substitution because Db does not belong to C major. However, the two chords do share a tritone interval.
A tritone interval is actually a diminished fifth (or augmented fourth) interval. This interval is made up of three whole steps, thus the name tritone. This substitution works because these two chords resolve to the tonic chord in similar ways.
In the key of C the dominant chord is Gdom7. The flatted fifth of Gdom7 is Dbdom7, which becomes the substitute chord. The example below shows that the second and fourth notes of these two dominant chords resolve to the first and second notes of the tonic chord by descending or ascending one half-step.
Another reason this substitution works, even though it is not diatonic, is that the root of the substitute chord which creates harmonic tension descends one half-step to the root of the tonic, creating a descending chromatic bass line. The tritone interval of the original dominant chord and its flatted-fifth substitute allow the tension created by these chords to be released by the tonic chord.
Another interesting characteristic about the tritone interval is that two tritone intervals added together produce an octave. As in the example shown above, when we go up the scale three whole steps from F (the second note of Dbdom7), we land on Cb (the fourth note of Dbdom7). When we go down three steps from F, we land on Cb an octave lower than the fourth note of the chord. Since Cb and B are enharmonic equivalent notes, you can see that Dbdom7 and Gdom7 share the same tritone interval from F to B and from B to F.