Throughout the 1940s he worked in a surrealist vein before developing his mature style. This has areas of pure color separated by thin vertical lines, or "zips" as Newman called them. In the first works featuring zips, the color fields are variegated, but later the colors are pure and flat. Newman himself thought that he reached his fully mature style with the Onement series (from 1948). The zips define the spatial structure of the painting, while simultaneously dividing and uniting the composition.