Atonality and polytonality

Tonal music is music in which the notes have a hierarchy: among them a centre of attraction, the tonic, can be told apart (Med 1996, ch. XV). All of tonality, with its degrees, its functions and its cadences, is organised around that single centre.

At the start of the twentieth century composers explored two opposite ways out of that hierarchy. One multiplies the centre: two or more keys sounding at the same time. The other removes it: no note rules the others.

Bitonality: two keys at once

Bitonality layers melodies or chords from two different keys, sounding at the same time (Med 1996, ch. XV). Each layer, on its own, is perfectly tonal. What is new is hearing them together.

The most famous example is in Stravinsky's ballet Petrushka: a C major arpeggio and an F sharp major arpeggio played together (Med 1996, ch. XV). Putting the two chords together on the keyboard:

Hear the notes

C major and F sharp major lie a tritone apart, and the two chords have no note in common. That is why each chord keeps its identity, and the ear hears two different brightnesses rather than a single chord. Separating the layers by register or by instrument, one low and one high, helps the ear follow both keys.

In the score each layer can carry its own key signature. Med's example puts a melody in A major, with three sharps, over another in B flat major, with two flats, each on its own staff (Med 1996, ch. XV).

Polytonality: several keys

Polytonality is the general name: several different keys at the same time, two or more. Med names among the best-known composers Béla Bartók, Darius Milhaud and Paul Hindemith (Med 1996, ch. XV).

Atonality: no centre

Atonality is the negation of tonality. It uses the twelve notes of the tempered chromatic system without relating them to any tonal centre, and it rejects the concepts of consonance and dissonance. Its best-known composer is Arnold Schoenberg (Med 1996, ch. XV).

An atonal line usually uses all twelve notes and keeps any one of them from settling. This one goes through all twelve without repeating one, and none of them sounds like an arrival:

Hear the passage

"Rejecting consonance and dissonance" does not mean everything sounds the same. It means that dissonance stops being a tension that asks to resolve into a consonance. In tonal music a dissonant chord is always on its way to another; in atonal music it can simply stay. Schoenberg called this the emancipation of dissonance.

What changes when there is no centre

Much of what one learns about tonality depends on the tonic, and stops working without it.

• There is no modulation. To modulate is to change tonic; if the music does without the tonic's function, there is nowhere to change from or to. Med notes it in his chapter on modulation: in atonal music modulation does not exist (Med 1996, ch. XXV).
• The key signature loses its point. A signature says which notes belong to the key. With no key there is nothing to say, and many composers write without a signature, putting each accidental before its note.
• Finding the main key is no longer possible. Lacerda teaches how to find a piece's key from its signature and the bass of its last chord, and warns that the procedure holds for music of the Classical and Romantic periods but not always for contemporary music, where many composers use no signature and some write with no tonal centre at all (Lacerda 1966, ch. XXVI).

Between the two extremes

Tonal, polytonal and atonal are not three sealed boxes. Much twentieth-century music sits in between: a centre that comes and goes, a tonal layer over a keyless background, a scale with no clear tonic such as the whole-tone scale. Tonality is one way of organising notes, and these are others, some with one centre more, some with one fewer.