Chromaticism or chromatic harmony is the use of chromatic notes and chords.
Chromatic harmony uses the twelve notes in the chromatic scale and gradually dispenses with the idea of key.
The idea of key is at the heart of a lot of melody and harmony. Music in a clear key is tonal music. Tonal music is based on the idea that a set of notes is chosen from the chromatic scale and ordered in some sort of logical way to produce a scale. A scale with a named tonic note forms a key. Notes that are part of the key are diatonic to that key, notes that are not are called chromatic.
This part of the guide concentrates on two scenarios that form the basis of chromatic harmony:
The first scenario, ambiguous key, is demonstrated in the chapters on polytonality and tone cluster. Polytonality is an interesting approach to harmony. Instead of abandoning key altogether, the idea is to use two or more keys simultaneously. A tone cluster is a harmonic idea that teeters on the edge of key. A tone cluster is a chord formed from consecutive notes in a scale, but it is quite difficult to tell whether a tone cluster is a diatonic chord or a chromatic chord.
The second scenario, absence of key, is atonality. Atonality is music without a key, it is the opposite of tonality. Atonality is demonstrated in the chapters on twelve tone harmony and serialism. Twelve tone harmony uses the 12 notes in the chromatic scale to write harmony without a key. Serialism extends the idea of randomness, which is inherent in twelve tone harmony, to select not only the pitch of a note, but also the other elements that go to make up a piece of music, in particular, rhythm and instrumentation.