What are the cosmic forces that govern music? The most important are the forces of tension (or instability) and release (or stability). These are the forces that move music forward in time. If we examine the forces at work within the basic major scale we can gain valuable insight into the inner workings of music. Let’s look at the major scale and decide which notes are stable and which are unstable and require resolution. The C major scale contains the notes C D E F G A and B. The notes that are stable are the three tones that make up the C major triad (C E G) or the first, third and fifth tones. These three tones occur as natural harmonics of a fundamental frequency and guitarists can confirm this by playing natural harmonics at fret twelve (the octave above the fundamental open string), fret seven (the fifth above the fundamental), and the fourth fret (the major third above the fundamental). Even if we don’t understand the acoustical reasons for this the stability of these tones is recognized by the ear. So if we agree that these notes are stable, then all others are unstable in various degrees and require resolution to the stable tones. Let’s look at the four remaining tones D F A and B and decide where we think they want to go. I think everyone would agree that the B has a very strong pull toward C. This is called the “leading tone” for that reason. Its relation to C by half-step is the reason for the strong pull. There is another half-step that exists between the fourth degree (F) and the stable third degree E. This causes the fourth degree (F) to be highly unstable wanting to move to E. Note that these two tones (F and B) together form a tritone which is a highly unstable interval that needs to resolve by either expanding to a sixth or contracting to a third. In our case the F would move to E and the B to C, resolving to the two stable tones of the tonic C major triad. This tritone can also resolve in the opposite direction where the F could move to F# and the B to A# resulting in a resolution to the root and third respectively of an F# major triad. This is known as the duality of the tritone and is the basis for the augmented sixth chord in classical theory and the jazz equivalent known as the tritone substitution. The two remaining tones D and A are relatively unstable but because of their relationship of a whole-step to the stable tones they are less active. The D could go either to C or E and the A most likely will want to move to G. So in summary:
b>C<d d>E<f G<a
Chords that contain the two most active tones (B and F) will have the greatest tendency to want to move back home to tonic (the three stable tones). These would be the dominant seventh chord (GBDF) and the leading tone triad (BDF). Since they have three notes in common let’s consider them the same as they are considered the same in actual practice. The I (tonic), iii (mediant), and vi (submediant) triads all share two stable tones and as a result are inactive and are considered to be slight variants of the tonic chord. Let’s consider these three chords as equivalents. That leaves the ii (supertonic) and IV (subdominant) triads. They are more active than the I, iii, and vi chords but less active than the V7 and vii chords. In practice they are used as a way of preparing the dominant (V7) chord so you tend to see them just before the V7 chord. These two also have two notes in common and are interchangeable in usage. In summary then we have three chord types: Tonic (I, iii and vi) Subdominant or Dominant preparation (ii and IV) and Dominant (V7 and vii). This may help us understand why certain chords sound as if they want to move to another and it makes sense that the most common chord progressions in music are IV-V7-I and ii-V7-I.
Introducing accidentals to create half-step relationships that do not exist in the diatonic scale should and does produce more tension. If we raise the second degree D to D# the ear will hear this as a high tension tone that has a strong pull toward E. If we lower the D to Db the ear will now want to resolve that tone to C. This is what is acting behind the scenes in the altered dominant chords in jazz. Take for example the G7 chord. We know that this chord already wants to resolve to tonic because it contains two very active tones (B and F). When we add the D# to this chord creating a G7#5 we now have an even stronger pull to tonic since the added note D# has a strong pull up to E (third of the tonic chord). What if we add the b9 to the G7 chord? The new note Ab is the lowered sixth scale degree and will want to resolve down to the note G (fifth of the tonic chord), again creating even greater tension. Adding the #9 (A#) to the G7 chord is an interesting case since the raised sixth degree (A#) will want to move by half-step up or down resolving to the B or A natural which would be the major seventh or sixth of the tonic C major chord. Adding the b5 (Db) to the G7 chord will create additional tension with the Db wanting to resolve to C (root of tonic) but in actual practice dominant seventh b5 chords are usually treated as tritone substitutions and will resolve in an entirely different way (down by half-step) which has to do with the duality of the tritone discussed earlier. I will take up that topic in greater detail in the future.
One other cosmic force to consider is the interval of a descending fifth. This force is also very strong. In tonal music it is realized through the dominant to tonic or the V-I progression which is considered to be the strongest of all harmonic progressions and the most important way to establish a key. It is also the basis for most chord progressions in jazz. Please see earlier article on harmonic progression for a more detailed explanation.
Of course these are general rules and music would be awfully dull if there weren’t many exceptions. Harmonic relationships that surprise are what really make things interesting!