Tonality defines a relationship between pitches in a musical composition. In conventional tonality, each pitch revolves around a central pitch, called the tonic. Each of the other pitches relates to the tonic in a specific way. The "key" of a piece defines this tonic note and the relationships around it. A piece in the key of C Major is based on a tonic of C.
Nearly all tonal music from the common practice period is arranged into scales of seven notes. Each note can be referred to by a roman numeral, in order to show the relationships around the tonic. Furthermore, each note in the scale can be referred to by function. The first note of the scale functions as the tonic. The breakdown of function and roman numeral in a major key is as follows:
In a minor key
, the VII functions as a sub-tonic
The function of these notes can be understood and felt. The tonic note and tonic chord have a feeling of resolution and lack harmonic tension. Each note has a different level of tension, a need to resolve back to the tonic. This tension is weakest in the mediants and strongest in the leading tone, since the leading tone is only a half-step away from the tonic. The dominant chord is the ideal place to set up a resolution, since it contains the notes with the most tension: the dominant, the leading tone, and the supertonic.
There are other forms of tonality that involve different methods of pitch organization, such as modality, pandiatonicism, polytonality, and quartal harmony. Atonality is the lack of organization around a central pitch.
The music of the last century-and-a-half has seen a steady departure from tonality as a compositional technique. Romantic composers often broke the "rules" stretching tonality as far it could go. Many modern composers explored atonality, even regarding it as the next logical step in musical development. We are, however, seeing a resurgence of tonality in contemporary music, sometimes referred to as neotonalism.