Consonance and Dissonance:   Notes that sound good together when played at the same time are said to be consonant.  Chords built only on consonances sound stable and pleasant.  You can listen to them for a long time without feeling the music needs to change to a different chord.  Notes played together that do not feel pleasant or sound harsh or unstable are said to be dissonant.  When you hear dissonance it usually wants to pull you back into consonance.  Of course terms like harsh or unstable are a matter or opinion but for this study we will look at generally accepted consonance and dissonance in western music.

If you have problems in tuning notes may not sound good together but this is not what consonance and dissonance are about.  Consonance and dissonance refer to intervals and chords.  The interval between two notes is the number of half steps.  Intervals have names that musicians commonly use like major third, perfect fifth, octave etc.  An interval is the measure between two notes.  Where there are more than two notes sounding it is called a chord.  Below are intervals that are considered to be consonant.

consonant intervals

In modern western music all of these intervals are considered to be pleasing to the ear. chords that contain these intervals are considered to be stable and restful, not needing to be resolved.  When we hear them we do not feel the need to go to another chord.

dissonant intervals

The intervals above are considered to be somewhat unpleasant or tension-producing.  In tonal music chords containing dissonances are considered to be unstable, when we hear them we expect then to move on to a more stable chord.  Moving from dissonance to consonance is called resolution or resolving the dissonance.  The pattern of tension and release caused by resolving dissonances is what makes music exciting.  Music that contains no dissonances can sound simplistic or boring.  On the other hand music that has many unresolved dissonances (twentieth century classical or "art" music) can be difficult for some people to listen to because of the unreleased tension.

resolving dissonance

Why are some note combinations consonant and some dissonant?  Preferences for certain sounds is partly cultural which is why traditional music of different cultures can sound so different.  Even in western music opinions about what is pleasant or not have changed over the centuries.  There is some physical basis as well in that the sound waves for consonant intervals fit together much better than dissonant ones.  At the end of the day what is pleasing or harsh or stable or unstable is primarily up to the individual listener.