Tonality is the organization of music around a fundamental note, known as the tonic note.1 A tonal composition feels tense and "in motion" until it concludes with the tonic note, at which point the composition feels resolved and stable. Atonal music, on the other hand, lacks a fundamental note.
Many systems of tonal music have evolved throughout the world. By far the most influential is major-minor tonality, which developed in Western Europe throughout the medieval and Renaissance eras, maturing in the Baroque period.
Major-minor tonality features a set of twenty-four scales (twelve major and twelve minor), each of which contains seven notes. The scale of C major, for instance, contains the notes C, D, E, F, G, A, B.
A chord can be constructed upon each of these seven notes. The C major chord, for instance, can be constructed upon the note "C" by adding the notes "E" and "G".
Thus, each scale features seven basic chords (one for each note of the scale). In the C major scale, the basic chords are C major, D minor, E minor, and so on.
The power of major-minor tonality lies in chord progressions. Within a given scale, some chords produce a feeling of stability and release, while others create a sense of movement and tension. The three fundamental chords of a given scale are the tonic (based on the first note of the scale), subdominant (based on the fourth note), and dominant (based on the fifth note).I178 In the C major scale, these chords would be C major (tonic), F major (subdominant), and G major (dominant).
Chord progressions typically focus on movement from one fundamental chord to another, whether directly (e.g. C major > G major) or indirectly via non-fundamental chords (e.g. C major > D minor > G major).I178
The chord progressions of major-minor tonality are capable of tremendous emotional range and power. It is therefore unsurprising that this system dominated Western art music for the Baroque, Classical, and Romantic eras. Although modern art music has largely departed from major-minor tonality, the system continues to flourish in virtually all film music and popular music.