Polytonality/bitonality

Definition

The use of two (bitonality) or more keys (polytonality) played or sung at the same time, e.g. the melody might be in the key of C major whilst the accompaniment might be in E major. This device was used by many 20th-century composers, eg Bartok, Ives, Holst and Stravinsky.

Example

Listen to this piece by Grainger which has a polytonal beginning.