One of the most important symbolic pairs is that of the dragon and the phoenix. Often depicted as though they are engaging in a heavenly ballet, they symbolize the harmonious relationship between the emperor (dragon) and empress (phoenix). The intertwined pair of celestial beings appears regularly on the interior oculus (center peak) of domes in Confucian and imperial temples especially. Dragon and phoenix also appear often on the rounded ends of roof tiles. Such symbols were not allowed as decoration on any but imperial structures. Since the emperor was considered the Son of Heaven (Tian Zi), you might expect various types of imagery associated with the firmament, and abstract cloud forms are among the favorites.