Oliver Neighbour in The New Grove, Second Viennese School
(Norton publisher) begins Chapter 2, Works, 1. Early Tonal Works, thusly:
Schoenberg's music may be divided into four periods, the second and third of which were inaugurated by crises in compositional technique that had important consequences not only for the composer's own work but for music in general. The music of the first period is tonal, or at least employs a tonality as a central point of reference. In 1908 Schoenberg abandoned tonality; he was the first composer to do so. The music of the ensuing second period is often called 'atonal'. Schoenberg considered this term nonsensical, preferring 'pantonal'. Since either term properly embraces his serial music as well, the period will be refered to here as 'expressionist'. From his work of this time he gradually evolved the principle of serialism, which he first used consistently in 1920; the serial music written between that date and 1936 constitutes the third period. The fourth, less well defined phase may be said to emerge during the 1930s. It is marked by greater stylistic diversity, including occasional returns to tonal composition.