Arnold Schoenberg (13 September 1874 - 13 July 1951) was an Austrian- born American composer, music theorist, teacher, writer, and painter. He is considered an important and influential composers of the 20th century.
Schoenberg was associated with the expressionist movement in German poetry and art. Schoenberg developed the twelve-tone technique where all twelve notes in the chromatic scale are ordered into a series. The group of his pupils and associates that followed his example of composition was called the Second Viennese School. The principal members were Schoenberg, Alban Berg, and Anton Webern.
Schoenberg’s works were labeled degenerate music by the Nazi party because they were modernist and atonal, and Schoenberg was Jewish. He then immigrated to the United States in 1933.
Below is an excerpt from Pierrot Lunaire, a melodrama by Schoenberg. Notice all the accidentals, a clue that this work is atonal.