Jazz harmony is based on the same axiom as Western classical harmony; generally, the idea that listening to music is more satisfying when harmony creates tension and then releases it through “resolution”. A song with only one chord will quickly get boring, so it’s important for composers and players to take the listener on a harmonic journey. A 12-bar blues song is a very short journey, but a sonically satisfying one because it uses the principle of tension and release.
We can see in the example below that there are only three distinct chords, C F and G. Each time we reach one of these chords, there’s a slight change in mood. We begin with the home chord of C (C-E-G), and we’re given four bars to get accustomed to the sound before we reach a turning point. The F chord brings us to a slightly different sonic space, but our comfortable home note of C carries over as the fifth of this chord (F-A-C). It’s not until the G chord (G-B-D) that we temporarily lose the sound of that C. Your ear wants to hear the B note move up a half step to good old familiar C, and this is the “tension”: that time spent expecting a resolution to familiar territory. When we move from G back to C, there is a feeling of closure, as if we’ve returned safely to the place we started. Creating this sense of resolution is how composers choose which chords will accompany a melody, and a jumping-off point for what comes next.
In our next example below, we see the same 12-bar blues structure, but we’ve changed the qualities of a couple chords to strengthen that sense of resolution. The G chord has been changed to a G7 (G-B-D-F), which builds extra tension by adding another note that wants to resolve a half step into the C chord. Now we have the third of our G chord (B) moving up to C, and the seventh (F) moving down to E. In jazz harmony we call this G7 the “five chord”, as it’s built on the fifth note of the C scale, and it represents this two-note resolution back to the home key. So, in the key of D, the “one chord” is D and the five chord is A7, in the key of Ab the five chord is Eb7, etc…
In the blues example we’re using, the F chord is an important destination. Why not emphasize this by arriving at F through its own five chord? In our C7 (C-E-G-Bb), the E moves resolves up to F and the Bb resolves down to A, providing the same function as our G7 to C. The chord progression below is common in many styles of music.
In jazz, the 12-bar blues usually adds a flatted seventh to all its important chords in order to “darken” the mood and avoid the sound of bright, happy major triads. It also means that both of the tension notes in our “five chord” resolve down a half step. As you can see below, the third of our C7 (E) resolves to the seventh of the F chord (Eb), while the seventh of our C7 (Bb) resolves to the third of F (A). These notes are what we call “guide tones”, as they guide the ear smoothly to a resolution while the root of the chord jumps around. If we kept moving those guide tones down another half step, the Eb and A of our F7 chord would become D and Ab, the third and seventh of a Bb7. Then we’d pass around the circle of fifths in a cycle, through Eb7 to Ab7 to Db7 and so on, the guide tones always moving down a half step, alternating between third and seventh of the chords.
But why stop there? In bars 7-12, instead of going straight from the C chord to the G7 and back, we can add an intermediary chord preceding G7 that helps to build tension more slowly, thus creating a more interesting journey. We know that our two important notes in G7 are the third (B) and the seventh (F). Our ears hear B as the stronger of the two, as it resolves directly to our home note of C. The F note resolving to E is less important, so a logical chord to precede G7 is one that has the tension note F while maintaining the consonant note C. Continuing with our guide tone theme of thirds resolving to sevenths and sevenths resolving to thirds, we’re looking for a chord, closely related to the key of C, whose third is F and whose seventh is C. Voila! D minor 7 is our answer.
Because this chord is built off the second note of the C scale, we can call it a “two chord” in the same way G is our “five chord”. This system of resolving is called a “two-five-one” progression, and it occurs in almost every jazz standard. A two-five-one in the key of Bb would be Cm7-F7-Bb. A two-five-one in E would be F#m7-B7-E, etc…
If preceding a target chord with its “five” creates satisfying tension and release, we can logically precede ANY chord with its five to make a progression more interesting without sounding random. In the case below, we’ve given the Dm7 its own five chord, A7. Although the short-term goal is to reach the D minor, it’s important to remember that the entire structure in bars 8-10 is just a complicated version of the resolution G7 to C.
Now we can merrily sprinkle new chords into our original blues progression, adding color and interest without losing our sense of tonality. In the next example, we’ve added a “two-five” leading to the F chord, then another two-five before the D minor. The A7 in bar 11 is just the five of D7, which is in turn the five of G7, which of course is the five of our home chord C7.
This is a basic tenet of jazz reharmonization: we can just keep inserting fives and two-fives in perpetuity, working backwards from our target chord. We could easily change the Gm7 in bar 4 to a G7, or the C in bar 11 to an E7, and our ears would be satisfied with the sound of the guide tones resolving by half-step. The chord progression below is a common one.
There’s another basic trick we can use to add interest. By now you may have realized that the guide tones of our five chords actually correspond to TWO different chords. The E and Bb of our C7 chord are also the seventh and third, respectively, of a Gb7 (F#7) chord. We can replace the bass note of our chord with one a tritone away, and the guide tone resolution stays the same. Our G7 (G-B-D-F) can now be played as a Db7 (Db-F-Ab-B). This other chord with its juxtaposed root can be called a “substitute five” or “sub five”. Where the five of F is C7, the sub five of F is Gb7.
Given all this, we can justify writing pompous progressions like this next one. We’ve gone from a three-chord blues to a nineteen-chord blues, and at this point our musical journey may feel like a trudge through deep snow. You can sense that this much harmonic information quickly becomes tedious, but short passages of this density aren’t uncommon, and the logic of it is sound. Bars 2-4 are a long resolution to F, and starting with the F minor we can see a string of guide tones that all stem from our original G7.
This way of working backwards from a target chord can be a good way to add interest and color to an otherwise simple chord progression. Adding (or implying) five chords and sub-five chords is a useful tool for soloists who want to play melodies that sit “outside” the changes but still make logical sense and resolve tension in a satisfying way. It is also imperative for jazz bass players to know how this works and what it sounds like, in order to better accompany a band.