Consonance and Dissonance

The definition of consonance and dissonance is pretty simple. An interval or chord that sounds good is called consonant. An interval or chord that doesn't sound good is called dissonant.

For an example, play a harmonic minor third interval. This sounds good, and therefore, is consonant. Now play the harmonic diminished fifth interval. This doesn't sound really good, and is therefore considered dissonant.

There is no general rule to determine which intervals and chords should be considered consonant and which dissonant. This depends largely on the people's taste. For example, the third intervals (major and minor thirds) were once considered dissonant, because they didn't sound good to people. But people's tastes changed over time, and now we feel these intervals are consonant.

Although I said that dissonant things sound bad, the fact is that dissonant things only sound bad when played alone. If they are a part of a peice of music, they may sound good.

Resolution

When we play something dissonant, it wants to move to something else. We must change to some other note or chord, whichever it is wanting to move to (which can be more than one), or it will sound bad. Remember when I said dissonant things don't sound bad when in a peice of music? This is exactly why. They change to wherever they are trying to change to.

The effect dissonance creates is called tension. When tension occurs, the music needs to change to some other note or chord. When we change over to that note or chord, it is said that the tension is resolved. This is called resolution.

Because tense (dissonant) notes want to change, they are also called "unstable".

For an example, play this:

E-------------------------------------|
B-------------0-----------------------|
G---------0-2-------------------------|
D---0-2-3-----------------------------|
A-3-----------------------------------|
E-------------------------------------|

This was the C major scale, but i haven't played the last C note. Right after the B note is played, do you hear how it wants to move to the C note? The 7th note of the major scale creates tension, and wants to resolve to the root note. Unless you play the C note at the end, the scale sounds incomplete.

Keeping the above discussion in mind, we can conclude that the definition of dissonance should not be that it "sounds unpleasant". Rather, dissonant should be defined as "unstable", that it wants to move to some other note or chord, and consonance as "at rest" or "stable", as it doesn't want to move to some other note.