The shadow wall has two special functions in quadrangle architecture. No matter how many quadrangles in old Beijing, even single courtyard, the street gate is open in the southeast. If we say the reason, we should start from Beijing's geographical location. Beijing is surrounded by mountains on three sides, with the Yanshan Mountains in the southwest, west, northwest, north and northeast. Judging from the topographic map, Beijing is also a city built halfway up the mountain. The mountainous areas in the west and north keep the cold air from Inner Mongolia and North China out of the mountains, while the warm and humid air currents from the south and southeast coasts often bring warm and humid air currents to Beijing. According to the five elements theory of the Book of Changes, the southeast belongs to dislocation, the five acts are wood, the attribute is wind, and the main news affairs. Taking gas from this direction is more conducive to the living of homeowners who live and work in peace and contentment than the south is dry and the east is vibrating. Therefore, it is not difficult to see from such a geographical environment and the orientation of the five elements that the most favorable orientation of the whole courtyard is the southeast. In this way, the quadrangle opened in the southeast will definitely have "qi" going straight to the south wall of the east wing. At this time, the shadow wall played a role in resisting the cold brought by the wind blowing directly on the gable. More importantly, the shadow wall is the first wall for tourists to look up when they enter the gate. Designers who pay attention to looking up and seeing happiness often use the shadow background wall to design auspicious patterns to achieve the purpose of beautification. In Wang Fu and the homes of some dignitaries in the city, many shadow walls are decorated with colored glazed tiles. What's more, tile fish pots are placed under the shadow walls, and ornamental lotus flowers are planted. koi fish and lotus flowers that reflect sunlight in the water are also quite exquisite.