Dissonance is the most hated, least understood, and most important aspect of music. Simply put, dissonance gives music its purpose, much like the villain in a movie. There would be no movie without the villain – at least not a good movie. This is true for music too, though to varying degrees and depending on the purpose you have in mind. It also depends on your understanding of dissonance.
Let’s begin with the chord. One chord holds consonance and dissonance. A C major chord, for example is built within the frame of a perfect fifth, less consonant that an octave, but still considered a perfect consonance. The note nestled in between the C and G that make up the fifth is an E, which, in this context is still considered by many to be consonant, though the major third between the C and E, and the minor third between the E and G are less consonant that the fifth. For the sake of example, we will also consider the thirds consonant entities.
In the key of C, there are two other major chords – F and G. Let’s move now from the C to the G. The G chord is built with the same relationships as the C – its notes are G – B – D. Once the G chord sounds, it is the same kind of consonance as the C chord. However, in relationship to the C it causes dissonance! Though we can’t hear the C chord in the physical world, our minds hold it, creating an implied dissonance. It’s this tension which is mostly just in our heads that, in my opinion, gives music purpose.
If you played the two chords together, you would hear a nice clash between them, especially between the B and C notes, which is a minor second (or major seventh), the harshest of our tonal dissonances. Because it is only an implied dissonance when the two chords don’t sound together, it doesn’t hit someone who hates dissonance the wrong way. It does, however, create a musical longing to return to the C chord. Even more so if you add the seventh to the G chord.
It’s the dissonance that makes our ears return to that C chord throughout the course of a piece of music. In most of the popular forms of music today, the relationships are pretty simple. The song writer starts with a chord that they usually return to again and again. Good examples are the standard 4-chord songs we hear. in C the most popular is C – G – Am – F. Right away we hear implied dissonance between the C and G. Moving from G to Am adds even more dissonance, though the move to Am actually brings us closer to C because the two chords share two notes (C has C, E, and G and Am has A, C, and E). From the Am we move to F which, in relation to the C causes as much tension as the G but it shares two notes with the Am (F has F, A, and C). When we finally get to C you can feel the relief.
Try it yourself. Play the four chords in a row. First, stop on the F chord. Pay attention to what happens in your mind. Do you finish it in your head by thinking a C?
Next, play it again and this time, add a C chord after the F. How does this feel?
This is a simple exercise, though many people aren’t consciously aware of the pull dissonance has. The clearest example of tension and release is Bdim to C. Play the notes B and F together a few times followed by C and E. This will do it.
I’ve just scratched the surface – entire college courses could be taught about dissonance. The best way I know to deeply understand it’s power is to experiment and trust your ears. If you really trust them, they won’t lie to you.