Inversion | 1. When a musical shape is mirrored.2. An inverted chord is formed when a note other than the root is in the bass. | Twentieth century. | Irregular metres | Often in modern or rhythmically based ethnic music, groupings of notes change, but the underlying pulse remains constant.
Groupings of two and three produce irregular accents and metres. | Twentieth century. | Microtone | Any interval noticeably smaller than a semitone, most often found in the music of Eastern European countries and also in Indian and Arabic music. It is sometimes used by contemporary classical composers. | Twentieth century. | Polytonality | The use of two or more keys played or sung at the same time, eg 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. | Twentieth century. | Retrograde | To go backwards. A melody or a section of music can be written or performed from the end to the beginning. The texture of the music including the harmonies can be written or performed from the end to the beginning. Retrograde inversion means the music can be written or performed backwards and upside-down at the same time. These are called serial techniques. | Twentieth century. | | |
| Serial | A 20th century method of musical composition invented by Schoenberg in which the 12 notes of the chromatic scale are organised into a series or tone row. This row can be transposed, inverted or played in retrograde, and forms the material basis for an entire work or movement. | Twentieth century. | Tritone | Interval of an augmented 4th. It is made up of 3 whole tones. | Twentieth century. |