Harmony is like a round-trip ticket. There are many methods of traveling to a destination and back again, and some transportation modes are more interesting than other. Likewise, a chord progression is a “route” that gets you from one musical place to another and back again—and there are ways to alter that route to make the musical journey richer and more interesting.
One method of developing richer-sounding harmony is with chord substitutions, where chords are replaced by other chords or additional chords are inserted into the progression. Chord substitution depends upon the fact that a chord (or any group of notes, for that matter) can be used in several different keys, and in each key it has a different function. There’s a lot of general information available on chord substitution, but for our purposes we’ll concentrate on the basic principles from the perspective of the bass.

One great thing about chord progressions is that they’re driven by the bass. The bass line has the power to deliver an implied tonality (key center) and still support the written (or traditional) chord progression. By way of example, lets look at the chords and melody of “Twinkle Twinkle Little Star.” As you’ll see, there are a lot of ways to imply chord substitutions in this simple tune—all by playing single-note bass lines.

Bassists often imply chord substitutions (some-times without knowing it) by altering the root movement of a chord progression. The new, or altered, chords replace or go between the original chords. One common root movement in jazz is a progression of fourths, where the chords move IIm7-V7-Imaj7. (It’s called a progression of fourths because each chord moves up a fourth to get to the next chord.) A progression of fourths can also be played with all dominant-7 chords although this is more commonly heard in blues music

Ex. 1 shows a reharmonization of “Twinkle Twinkle” using a progression of fourths. In bar 2 an A7 chord is implied by the A in the bass—but it still fits the original F chord, because A is a chord tone (the 3rd) in an F chord. From there, the new progression moves up by fourths: first to Dm7, then to G/B and G7, and finally back home to C.

You can see and hear how a progression of fourths has more energy than the original changes. Much of this comes from the dominant-7 chords. Each 7 chord contains a tritone between it’s 3rd and 7th, and since the tritone is a highly “active” or “energetic” interval, the dominant 7 is the chord that turns the energy of the progression back toward the tonic.

A bass line that moves chromatically generates a leading-tone feel between the chords where one might not otherwise be expected. Depending on the melody, chromatic root movement can imply either a diminished chord or an augmented chord. Augmented and diminished chords are symmetrical—all the intervals within them are either all minor 3rds (in the case of diminished chords) or major 3rds (augmented chords). As a result, they each have a lot of “stationary” energy, which is released when the harmony moves a half-step either way. In Ex. 2 the C#dim chord releases energy when it moves to the Dm7, as does the D#aug when it moves to the F. (Play the chords on a keyboard and you’ll hear what I mean.)

Instead of moving chromatically, a substituted chord progression can move along in the steps of a diatonic scale, such as the major scale. This type of substitution is very useful because you can imply any chord throughout the diatonic structure. In Ex. 3, the roots of the new progression move along the C major scale to G7.

Perhaps the most common substitution used by bass players is tritone substitution. (A tritone is the interval of an augmented fourth or diminished fifth.) What most players don’t understand is why it works. Typically, it’s used in an IIm-V7-I progression, where the V7 chord is replaced with a 7 chord a tritone away, creating a powerful chromatic bass line combined with the energy of a 7 chord. In Ex. 4, the chord in question is the G7, which is replaced by Db7. If you spell out each chord (G, B, D, F for G7 and Db, F and Ab, B for Db7), you can see that they have two notes in common: F and B. That’s why the substitution works—even though the chord names are different, the two chords sound similar (but not identical) because of the notes they share.

Western music is built upon “tertian harmony,” or harmony based on thirds. These thirds consist of every other note of a diatonic scale, and they alternate as major and minor thirds. This is the raw material from which we build chords. All diatonic structures utilize the same sequence of alphabetic thirds: in the key of C, the sequence is C, E, G, B, D, F, and A. (See Fig. 1.) To spell any chord, all that’s necessary is to plug in the appropriate sharps or flats to suit the key or diatonic structure in which you‘re working. Because of the simple sequence of alternating major and minor thirds, even complex chords tend to overlap heavily, creating multiple relationships between them. As a result, you can come up with various chord substitutions just by adding thirds underneath a chord, or by subtracting them.

Ex. 5 shows the diatonic structure of C major organized in three groups: IIm7 chords, V7 chords, and Imaj7 chords. As I mentioned earlier, IIm-V-I is a standard jazz chord progression, so it makes sense to use it as a point of departure when discussing harmony. The remaining diatonic chords in Ex. 5 are possible substitutions for the IIm7, V7, and Imaj7 chords. For example, because a IIIm7 chord shares three notes with a Imaj7 chord, Em7 can be used in place of Cmaj7. Likewise, Bm7b5 can be used in place of G7.

Ex. 6 shows how this kind of substitution can be performed. In this case, the 3rd above each root of each chord; therefore, Dm7-G7-C becomes F-Bm7b5-Em7. Using a combination of the original and substitute chord changes, we can effectively keep the original IIm-V-I chord vamp from becoming monotonous by implying substitute chord changes with the bass in the second two bars.

Minor-7 can belong to three different keys. Consider a reharmonization of bar 1 of “Twinkle Twinkle” in which the tonic, C, moves to Em7 (Ex. 7). By playing a pedal tone (holding the C in the bass), you can create an extended chord: C9maj7, which is essentially Em7 with C in the bass. Likewise, if you played an A under the Em7 chord, you’d be implying an Am11 chord, where the G, B, and D acted as the 7th, 9th, and 11th chord tones, respectively.

Ex. 8 shows a good substitution for making a rock or blues progression sound jazzy. A basic difference between jazz and rock or blues is that the typical jazz progression moves IIm-V-I, while rock and blues progressions move IV-V-I. Fortunately, IIm and IV chords are interchangeable. By playing a D under the F chord in Ex. 8, you imply a Dm7 chord—instantly adding a subtle change to the “color” of the harmony at that moment.

In Ex. 9, by playing the 5th of the tonic (G), you can change a Bm7b5 chord into a G9. This substitution doesn’t produce a large contrast in energy; both chords gravitate towards the tonic and contain a tritone. What is different (and useful) is the root movement of each chord. The original Bm7b5 moves by a minor second to the tonic, which is very smooth; the substitute G9, however, leaps a perfect fifth. This channels the flow of the energy into the bass—a technique bass players often use in walking lines.

As you can see, we bass players don’t have to play chords—we imply them with the lines we choose. Strangely enough, this perspective isn’t commonly taught, so many bassists tend to restrict their thinking to simple lines that strictly adhere to the chords played by the other instruments. For those players, the bass is little more than a pitched bass drum—a bleak situation indeed for an active mind!