The key system of tonal music depends in the first place upon being able to establish a particular point on the circle of fifths as being a stable tonal center. This tonal center is the keynote by which the key is named. The chord that takes that keynote as the root is aptly named the tonic chord, which means that the tonic chord itself represents and supports that tonal center or keynote. Within any chord progression in that key, the tonic chord will represent the central chord around which the other chords of the key will revolve. And this is where the idea of functional harmony comes in. Each chord performs a role in establishing and maintaining the tonic chord as the tonal center of the music.
Music that uses the key system in this way is called tonal music in that it is music that is built up and organized around a particular tonal center [modal music looks beyond the major and minor key system of classical music and uses other musical scales and modes, such as the system of church modes that preceded the adaption of our major and minor scales. Other harmonic systems are Chord-scale theory, (the chord is the scale, and the scale is the chord), Jazz harmony, Pandiatonic harmony, Use of compound chords and split chordal streams, Bitonality and polytonality, Use of different types of chord shapes, such as quartal (built up from fourths) and quintal (built up from fifths) harmony, Use of tone clusters and other alternative intonation models].