Arnold Schoenberg has exercised very considerable influence over the course of music in the 20th century, particularly through his development and promulgation of theories of composition in which unity in a work is provided by the use of a determined series, usually consisting of the twelve possible different semitones, their order also inverted or taken in retrograde form, and in transposed versions. Schoenberg's earlier compositions are post-romantic in character, followed by a period in which he developed his theories of atonality, music without a key or tonal center. Born in Vienna in 1874, he spent his early career in Berlin, until the rise to power of Hitler made it necessary to leave Germany and find safety in America, where he died in 1951. With his pupils Anton Webern and Alban Berg, both of whom he outlived, he represents a group of composers known as the Second Viennese School.