Harmony

Harmony helps musicians predict, recreate,
and exploit a variety of musical experiences, such as:

The "sadness" or "happiness" of a melody or chord,
Consonance and dissonance (sour notes),
Tonality or the "gravity" of pitches and chords in relation to others,
"Tension and resolution". 

Understanding harmony can open exciting new avenues
for improvising, composing and arranging.