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.
Thinking in Music 
Hey!
I’m not sure I agree with your interpretation of dissonance; what you have described, with C chord moving to the G chord, and the G chord feeling tense because of the mind holding the C chord is, I think, just part of an intrinsic functional harmonic system that implies and necessitates resolution. The leading note, the third of G, requires movement upwards by a semitone to the C not becuase of dissonance, but becuase of the human ear’s need for resolution. This is very different I think.
To say that a dissonance exists due to the major and minor thirds is incorrect I think. The ratios that govern these intervals are very pure (1/4 and 1/5), and they are both prevelant low down in the harmonic series (which shows us why triadic harmony makes sense to the brain). I think you are right in saying that dissonance gives consonance meaning, but in this very simple triadic example there is no dissonance.
A better example would maybe be this: If we take your given perfect cadence and add to the Dominant chord (the chord of G) its 7th (an f), the flat and sharp nine’s (Ab and A#), and maybe a flat13 (an Eb) and #11 (a C#) to create a Galt chord (something very common in certain styles of Jazz) the ratios inbetween all these notes are much more impure than the pure triad, the ratios are not as nice and clean as 1/4 or 1/5, and this would create a dissonance, which would then resolve nicely to a Cmaj7#11 maybe. What do you think?
F
I agree with you. The reason i think our ear seeks that resolution is implied dissonance. The progressions we’re accustomed to in western music originated in a climate of the rejection of dissonance, pre-medeival. I think most of what we do on this planet is like that, from music to the space shuttle (it’s size related to the width of chariot wheels). We build on the old and forget our origins… Hmm, sorry to digress!
I think I disagree. I don’t think dissonance exists within a functional harmonic progression, but i DO agree that we build on our origins in so much that what is said to be dissonance changes. 200 hundred years ago the tritone was said to be dissonant, but our 21st century ears have got used to it, and no longer find it so jarring.
I reckon that dissonance exists outside of function, that dissonance can only exist chord by chord; a progression cannot be said to be dissonant, only a chord. for example, a progression of triadic chords cannot be said to be dissonant. C – Db – Gb – Bb cannot be said to be dissonant because the relationship within each given triad is so strong, it is all 1/3, 1/4 and 1/5 ratios. Maybe in the distant fog of the past it would offend the ear, but not now. I understand what you’re saying, that chords may be irrationaly intervalic when compared to the root chord, but you cannot have a dissonance between two sounds heard at different times. function, like the connection between the leading tone to tonic exist becuase of the human brain’s need for symetry and order, not becuase of dissonance.
I reckon to have dissonance, one must hear notes at the same time. if you have a triad of C and superimpose upon it an impure ratio, like a Db or something, that will jar the ear, due to the lack of pure ratio.
Hmmm… the ratios Pythagoras discovered that lead to just intonation were a pretty great discovery. We move from the simpler frequencies of perfect consonance through imperfect consonance to mild dissonance and sharp dissonance. Imperfect consonance is less consonant (or more dissonant) than the perfect consonance. Anything less than perfect could be considered dissonant (I can already see your conservatory trained head shaking!). And depending on the context, even a consonance could need resolution – a tension that I equate with a cognitive perception of past tones. Though, I do agree with you that using functional harmony, you can lead the ear to a completely different key that was never even hinted at previously in the piece, I still believe it’s a matter of our musical memory that creates the feeling of relief we get on arrival. We all remember the relationships that lead us there from past pieces.
I totally disagree with you that something like I – IV – V – I is free of dissonance. On the contrary, I think it’s the tension created by our psychological perception of dissonance that makes this kind of functional harmony satisfying!
Thorne Palmer is absolutely correct. Dissonance is WHY the brain feels the need for resolution. The contrast of dissonance and consonance is one of the most important underlying concepts in music theory. Without it, “resolution” is merely ad hoc and without a basis in physics or psychoacoustics. Please read the peer-reviewed literature on music theory if you think otherwise.
I play a bit of keyboards. It’s an interesting harmony.
The E is slightly dissonant with C, but our Western ears have been trained to accept it as consonant. The undertone of a C and E produces a C two octaves lower (as does an E and a G), but in our Western even tempered intonation world, the undertone C’s are out of tune, whereas in just intonation, they would be a perfectly tuned C.
Right! The same is true of equal temperament. Forcing the half steps to be equal lessened the purity of the true perfect fifth, but it’s functional benefits outweighed the impurity of sound!