As early as the Neolithic period, a basic principle of Chinese architecture was already established, wherein columns spaced at intervals, rather than walls, provided the support for the roof. Walls came to serve merely as enclosing screens. Although the typical Chinese roof was probably developed in the Shang (c.1523–1027 B.C.) or the Chou (1027–c.256 B.C.) period, its features are unknown to us until the Han dynasty. Then it appeared in the form that we recognize today as a hallmark of Chinese architecture—a graceful, overhanging roof, sometimes in several tiers, with upturned eaves. The roof rests on a series of four-part brackets, which in turn are supported by other clusters of brackets set on columns. Decorative possibilities were soon realized in the colorful glazed tiling of roofs and the carving and painting of brackets, which became more and more elaborate.